It keeps throwing this error but there is nothing wrong with config.
Failed to initialize plugin 'QuickSmelt v4.0.5' (JsonReaderException: Additional text encountered after finished reading JSON content: ,. Path '', line 11, position 3.)

That actually does indicate an error with either your config or data file for the plugin, but hard to tell which exactly with only seeing the first line.

{
"SmeltSpeed": 20,
"WoodRate": 2,
"CharcoalRate": 1.5,
"CanCookFoodInFurnace": false,
"UsePermissions": false,
"Large Furnace Multiplier": 4.0,
"Campfire Multiplier": 3.0,
"Oil Refinery Multiplier": 4.0,
"Water Purifier Multiplier": 4.0
},

Merged post

Thats the config

I think you must remove , in the end