"The script needs to derive from MonoBehaviour"

Hi,
I am getting this error in a new project When I try to add a
component to the main camara. And if I create a new component the "public"
variables I declare dosen`t appear in the inspector.

 

There are other compile errors, this is visible at the bottom of the screen. Fix those first, and it should work.

 

I came here looking for a solution. Finally, I solved this problem using the Help menu and then Reset Packages to defaults.