Plugin BetterChat is using Oxide naming, please update to uMod naming
Error while compiling: BetterChat.cs(786,69): error CS0103: The name `Core' does not exist in the current context

Any chance of a developmental version of the plugin?

Should work with a minor change to remove "Core." from a couple locations:

private void LoadData<T>(ref T data, string filename = null) => data = Interface.Oxide.DataFileSystem.ReadObject<T>(filename ?? DataFileName);
private void SaveData<T>(T data, string filename = null) => Interface.Oxide.DataFileSystem.WriteObject(filename ?? DataFileName, data);​