Copilation error after adding namespaces.

So for starters, in the beginning my game worked fine. But then I added a new script, and if I try to use

using UnityEngine.SceneManager;

I get an compilation error “All compilation errors have to be fixed bevor entering game mode”.
I tried deleting the cript and creating a new one and if I dont edit it, it works fine, but if I try to use the namespace I get an error. I dont use the sript anywhere. Also I used the same Namespace in another script in the same Game, and there it is fine. This also happens if I try adding
but not if I do stuff like creating an integer.

The correct namespace is UnityEngine.SceneManagement;

Then , you will be able to call SceneManager.LoadScene