Adding reference to Nunit Test project - "operation could not be completed" error

I use VS 2017. I have created Nunit.Test project, and I want to add reference to my main project, of which methods I want to test. When I am trying "Add > Reference" an error occurs, with dialog:

The operation could not be completed.

I tried to check versions of Nunit or reinstall it, but it did't help. Some ideas?