How to fix error CS0122 is not available due to its protection level?

Question

.NET

Good afternoon, please tell me, I decided to merge two c# projects (the language version is the same and the compiler is the same). In one project, such an error is not observed, and in another it arises for unknown reasons, what is the reason and how to fix it - after all, in one project the error does not pop up?! Thanks in advance.

Language version 9.0
.NET version 5.0.

Answer 1

StrongReference must be declared public or InternalsVisibleTo must be set.
Same with fields.