MSTest cannot find TestAdapter.dll

For me, clearing the VisualStudioTestExplorerExtensions folder and restarting Visual Studio (as per this github issue) did the trick. In my case, it contained two folders for different versions of MSTest.TestAdapter, I don't know whether or not that's relevant.

I spent the whole last week trying to solve the same issue. Only yesterday did I realize that running tests from the console is a time saver. Much to my surprise, I've just moved all my project -which was inside a "OneDrive" folder, to a different location. Now, it works like a charm! All my tests run from the Visual Studio IDE. I don't know if moving the files had anything to do with it, but you can give it a try.

Reinstalling all solution's Nuget packages fixed this issue for me.

Please run this command on the package console:

Update-Package -reinstall

then try again, or restart VS, then try again.

Reference: https://ardalis.com/force-nuget-to-reinstall-packages-without-updating/

For anyone else who stumbles on this question, the problem was caused because a few of my test projects didn't have the MSTest.TestAdapter NuGet package included. I added the MSTest.TestAdapter package to all of my test projects, and the problem seems to be solved permanently now.