Webtest - missing dll

When I creating webtests I want to add "Web Test Plug-in" I get message box with information that one of my nuget library is missing.

Whole my solution is compilable and this "missing" library exists as entry in csproj and config files. After compilation of test project this library is inside "bIn" folder. I check under dotPeek version, publicToken etc is correct.

My question is where webtest project look for this library?