(02:10:36) | Failed to call hook 'ChatCMD' on plugin 'DiscordRewards v1.3.1' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.DiscordRewards.<RandomString>m__1 (System.String s) [0x00000] in <6e3df63345e84d75be091cb83b2b326b>:0 
  at System.Linq.Enumerable+SelectIPartitionIterator`2[TSource,TResult].PreallocatingToArray (System.Int32 count) [0x0001e] in <351e49e2a5bf4fd6beabb458ce2255f3>:0 
  at System.Linq.Enumerable+SelectIPartitionIterator`2[TSource,TResult].ToArray () [0x00023] in <351e49e2a5bf4fd6beabb458ce2255f3>:0 
  at System.Linq.Enumerable.ToArray[TSource] (System.Collections.Generic.IEnumerable`1[T] source) [0x0001f] in <351e49e2a5bf4fd6beabb458ce2255f3>:0 
  at Oxide.Plugins.DiscordRewards.RandomString (System.Int32 length) [0x00022] in <6e3df63345e84d75be091cb83b2b326b>:0 
  at Oxide.Plugins.DiscordRewards.ChatCMD (Oxide.Core.Libraries.Covalence.IPlayer player) [0x0010e] in <6e3df63345e84d75be091cb83b2b326b>:0 
  at Oxide.Plugins.DiscordRewards.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x0028f] in <6e3df63345e84d75be091cb83b2b326b>:0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <70dc8de1abd4423db5f501c6845edf22>:0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <ac41dd3599754d448b8c218b34645820>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <ac41dd3599754d448b8c218b34645820>:0

I've checked my bot keys and CFG, nothing wrong with it, and also verify at JSONLint. Please advice, Thanks!

Manage to solve it. Had to delete the old CFG.