How to solve errors when building a Unity project. error cs0246?

Question

C++ / C#

Unity, as I understand it, complains about the NUnit package in scripts, but there is no way without these packages. Screenshots are shown below.

Answer 1

to check up for what in general is necessary .
Most likely it goes as a library (for example, for Windows, or Android), in the project, select this library to put the platforms where the build does.
Check whether this library is generally suitable for the platform on which you are building.

Answer 2

Find and connect the NUnit lib. But I don't see something in the code why you need it.