I don't know why it's greyed out like that

Harmless error, should only occur once. Will be handled better in a future version.

Merged post

Should be fixed in v4.1.0

So I tried a fresh install first and a got this 

 Loaded plugin Stack Size Controller v4.0.0 by AnExiledDev
(05:23:06) | [Stack Size Controller] Populating Individual Item Stack Sizes in configuration.
(05:23:06) | [Stack Size Controller] Exception encountered during GetStackSize. Item: firework.boomer.pattern Ex:System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
  at System.Collections.Generic.Dictionary`2[TKey,TValue].get_Item (TKey key) [0x0001e] in <eae584ce26bc40229c1b1aa476bfa589>:0 
  at Oxide.Plugins.StackSizeController.GetStackSize (ItemDefinition itemDefinition) [0x00023] in <b451b0ad89fc4a5698712a9dc7a1e15e>:0
(05:23:06) | [Stack Size Controller] Exception encountered during GetStackSize. Item: xmas.advent Ex:System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
  at System.Collections.Generic.Dictionary`2[TKey,TValue].get_Item (TKey key) [0x0001e] in <eae584ce26bc40229c1b1aa476bfa589>:0 
  at Oxide.Plugins.StackSizeController.GetStackSize (ItemDefinition itemDefinition) [0x00023] in <b451b0ad89fc4a5698712a9dc7a1e15e>:0
(05:23:06) | [Stack Size Controller] Exception encountered during GetStackSize. Item: xmas.double.door.garland Ex:System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
  at System.Collections.Generic.Dictionary`2[TKey,TValue].get_Item (TKey key) [0x0001e] in <eae584ce26bc40229c1b1aa476bfa589>:0 
  at Oxide.Plugins.StackSizeController.GetStackSize (ItemDefinition itemDefinition) [0x00023] in <b451b0ad89fc4a5698712a9dc7a1e15e>:0
(05:23:06) | [Stack Size Controller] Exception encountered during GetStackSize. Item: attire.snowman.helmet Ex:System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
  at System.Collections.Generic.Dictionary`2[TKey,TValue].get_Item (TKey key) [0x0001e] in <eae584ce26bc40229c1b1aa476bfa589>:0 
  at Oxide.Plugins.StackSizeController.GetStackSize (ItemDefinition itemDefinition) [0x00023] in <b451b0ad89fc4a5698712a9dc7a1e15e>:0
(05:23:07) | [Stack Size Controller] Reverting stack sizes to vanilla defaults.
(05:23:07) | Unloaded plugin Stack Size Controller v4.0.0 by AnExiledDev​

I also tried to install by overlaying the old and removed the data files except the vanilla files and got 

Web request callback raised an exception (JsonReaderException: Unexpected character encountered while parsing value: <. Path '', line 0, position 0.)
  at Newtonsoft.Json.JsonTextReader.ParseValue () [0x002b3] in <2ae3f091decd4989b3c4d466e6eb5985>:0 
  at Newtonsoft.Json.JsonTextReader.Read () [0x0004c] in <2ae3f091decd4989b3c4d466e6eb5985>:0 
  at Newtonsoft.Json.JsonReader.ReadAndMoveToContent () [0x00000] in <2ae3f091decd4989b3c4d466e6eb5985>:0 
  at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.ReadForType (Newtonsoft.Json.JsonReader reader, Newtonsoft.Json.Serialization.JsonContract contract, System.Boolean hasConverter) [0x00043] in <2ae3f091decd4989b3c4d466e6eb5985>:0 
  at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize (Newtonsoft.Json.JsonReader reader, System.Type objectType, System.Boolean checkAdditionalContent) [0x000d9] in <2ae3f091decd4989b3c4d466e6eb5985>:0 
  at Newtonsoft.Json.JsonSerializer.DeserializeInternal (Newtonsoft.Json.JsonReader reader, System.Type objectType) [0x00053] in <2ae3f091decd4989b3c4d466e6eb5985>:0 
  at Newtonsoft.Json.JsonSerializer.Deserialize (Newtonsoft.Json.JsonReader reader, System.Type objectType) [0x00000] in <2ae3f091decd4989b3c4d466e6eb5985>:0 
  at Newtonsoft.Json.JsonConvert.DeserializeObject (System.String value, System.Type type, Newtonsoft.Json.JsonSerializerSettings settings) [0x0002d] in <2ae3f091decd4989b3c4d466e6eb5985>:0 
  at Newtonsoft.Json.JsonConvert.DeserializeObject[T] (System.String value, Newtonsoft.Json.JsonSerializerSettings settings) [0x00000] in <2ae3f091decd4989b3c4d466e6eb5985>:0 
  at Newtonsoft.Json.JsonConvert.DeserializeObject[T] (System.String value) [0x00000] in <2ae3f091decd4989b3c4d466e6eb5985>:0 
  at Oxide.Plugins.StackSizeController.SetVanillaDefaults (System.Int32 code, System.String response) [0x0002d] in <b451b0ad89fc4a5698712a9dc7a1e15e>:0 
  at Oxide.Core.Libraries.WebRequests+WebRequest.<OnComplete>b__42_0 () [0x00034] in <9882f28dc2204b4dba514a9ad18f5042>:0
 [Stack Size Controller] Reverting stack sizes to vanilla defaults.
(05:21:46) | Failed to call hook 'Unload' on plugin 'StackSizeController v4.0.0' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.StackSizeController.GetVanillaStackSize (ItemDefinition itemDefinition) [0x00000] in <b451b0ad89fc4a5698712a9dc7a1e15e>:0 
  at Oxide.Plugins.StackSizeController.RevertStackSizes () [0x00069] in <b451b0ad89fc4a5698712a9dc7a1e15e>:0 
  at Oxide.Plugins.StackSizeController.Unload () [0x00010] in <b451b0ad89fc4a5698712a9dc7a1e15e>:0 
  at Oxide.Plugins.StackSizeController.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x000c8] in <b451b0ad89fc4a5698712a9dc7a1e15e>:0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <60c318df79ed41688ea59335e48d61ad>:0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <9882f28dc2204b4dba514a9ad18f5042>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <9882f28dc2204b4dba514a9ad18f5042>:0
(05:21:46) | Unloaded plugin Stack Size Controller v4.0.0 by AnExiledDev

When loaded, it loads the same config as the previous version no stack sizes in the config meaning I cant load the size's that I use on my server 

I had to revert back to 3.4.3 that one works flawlessly

Again I tried a fresh install 1st error and 2nd tried an install overlay 2nd error and got both of these errors.
I tried both ways to get this to work but if its not even working on a fresh install theres a problem

And before anyone asks a dumb question:
Yes I am the server owner
Yes I know what I'm doing
Yes I configure my own server so I have the jist of what plugins do 
Yes I turned it off and back on again (the server) :)

This one works fine. Those errors have been reported previously in several threads, goes away after reload and effects nothing. 

Looks like your vanilla defaults has been modified and are corrupt. Delete every datafile, backup and delete config, and plugin file. Unload plugin first. 

Once you've confirmed everything is a clean slate, load the plugin, then to be extra sure reload one more time. You shouldn't get any errors except item missing. You should get messages about downloading vanilla defaults, and populating IndividualItemStackSize in configuration. 

To clarify, the first errors you provided are harmless and fixed by a plugin reload. 
Second error indicates the vanilla defaults file is somehow corrupted, or unfinished. Perhaps a partial download, it was modified for some reason or was interrupted during file write.

You're post is amusing since you're basically saying I didn't test my plugin before release when your environment is the issue. Relax.

LOL no not at all bro I didnt mean for it to sound like that my bad

Sorry just misunderstood. If you want to try v4.1.0 it'll fix the first errors (I think, I stopped getting them afterwards, but I didn't think that would fix it honestly) but the second error should be an invalid datafile for vanilla defaults. If every file is deleted it should work if the plugin is loaded then.

[Stack Size Controller] Exception encountered during GetStackSize. Item: blueprintbase Ex:System.NullReferenceException: Object reference not set to an instance of an object
  at Oxide.Plugins.StackSizeController.GetVanillaStackSize (ItemDefinition itemDefinition) [0x00000] in <1e1dbcf99a634fe09a0f09a80f36d789>:0

  at Oxide.Plugins.StackSizeController.GetStackSize (ItemDefinition itemDefinition) [0x00023] in <1e1dbcf99a634fe09a0f09a80f36d789>:0
 
(Filename: ./Runtime/Export/Debug/Debug.bindings.h Line: 39)
Failed to call hook 'OnMaxStackable' on plugin 'StackSizeController v4.1.0' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.StackSizeController.GetVanillaStackSize (ItemDefinition itemDefinition) [0x00000] in <1e1dbcf99a634fe09a0f09a80f36d789>:0

  at Oxide.Plugins.StackSizeController.GetStackSize (ItemDefinition itemDefinition) [0x001e9] in <1e1dbcf99a634fe09a0f09a80f36d789>:0

  at Oxide.Plugins.StackSizeController.OnMaxStackable (Item item) [0x00000] in <1e1dbcf99a634fe09a0f09a80f36d789>:0

  at Oxide.Plugins.StackSizeController.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00efc] in <1e1dbcf99a634fe09a0f09a80f36d789>:0

  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <60c318df79ed41688ea59335e48d61ad>:0

  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <9882f28dc2204b4dba514a9ad18f5042>:0

  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <9882f28dc2204b4dba514a9ad18f5042>:0
 
(Filename: ./Runtime/Export/Debug/Debug.bindings.h Line: 39)
[Stack Size Controller] Exception encountered during GetStackSize. Item: blueprintbase Ex:System.NullReferenceException: Object reference not set to an instance of an object
  at Oxide.Plugins.StackSizeController.GetVanillaStackSize (ItemDefinition itemDefinition) [0x00000] in <1e1dbcf99a634fe09a0f09a80f36d789>:0

  at Oxide.Plugins.StackSizeController.GetStackSize (ItemDefinition itemDefinition) [0x00023] in <1e1dbcf99a634fe09a0f09a80f36d789>:0
 
(Filename: ./Runtime/Export/Debug/Debug.bindings.h Line: 39)
Failed to call hook 'OnMaxStackable' on plugin 'StackSizeController v4.1.0' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.StackSizeController.GetVanillaStackSize (ItemDefinition itemDefinition) [0x00000] in <1e1dbcf99a634fe09a0f09a80f36d789>:0

  at Oxide.Plugins.StackSizeController.GetStackSize (ItemDefinition itemDefinition) [0x001e9] in <1e1dbcf99a634fe09a0f09a80f36d789>:0

  at Oxide.Plugins.StackSizeController.OnMaxStackable (Item item) [0x00000] in <1e1dbcf99a634fe09a0f09a80f36d789>:0

  at Oxide.Plugins.StackSizeController.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00efc] in <1e1dbcf99a634fe09a0f09a80f36d789>:0

  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <60c318df79ed41688ea59335e48d61ad>:0

  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <9882f28dc2204b4dba514a9ad18f5042>:0

  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <9882f28dc2204b4dba514a9ad18f5042>:0
 
(Filename: ./Runtime/Export/Debug/Debug.bindings.h Line: 39)

Rollback to v4.0 That version works fine. And if you don't like the config setup please use another plugin. Everyone moans everytime it changes and I no longer care, it'll remain in the configuration in it's current state for as long as I own it.