No scripts derive from monobehaviour.

For whatever reason, I can't use any scripts at all in Unity. Every time I try to add a script to an object, I get the following error. Can't add script behavior TMP_CoroutineTween. The script needs to derive from MonoBehaviour! Occasionally the error will be different but all of them point to not being able to derive from MonoBehaviour. Even prefabs with scripts already attached just won't compile. This is not related to any particular script or project. This even happens in Unity's 2D Game Kit so I know it is not related to the code I've written. I have tried reinstalling unity, running it as an admin, deleting projects, and double checking class names and code. I am running out of ideas. Could I be installing it wrong somehow? Thanks in advance for any advice.