It was working fine in a mod pack I made then when I tried to add the mod individually to the forge (not in a mod pack) it came up with this message 'Earth2Java (earthtojavamobs) has failed to load correctly java.lang.NoClassDefFoundError: me/shedaniel/autoconfig/ConfigData

Why did this happen and what can I do to fix it? (Or the mod developers can try to fix it.)

It require cloth config api, should be set as required dependency already.
From where you have downloaded it, CurseForge or Modrinth?