The type or namespace name 'NUnit' could not be found | Unity

Question

i have next errors:

The type or namespace name 'NUnit' could not be found (are you missing a using directive or an assembly reference?)
The type or namespace name 'UnitySetUpAttribute' could not be found (are you missing a using directive or an assembly reference?)
The type or namespace name 'UnitySetUp' could not be found (are you missing a using directive or an assembly reference?)
The type or namespace name 'TestAttribute' could not be found (are you missing a using directive or an assembly reference?)

More under a page:

enter image description here

Code: https://pastebin.com/5zTjwPZw

How fix it?

Answer 1

I had the same issue in a custom package after updating to 2020.2.3f1.

In the case of packages (or if you are using assemblies), they now seem to enforce that Editor's only assemblies should be marked as such in the Platforms section of the inspector: