Type universe cannot resolve assmebly

I write some unit test using MSTest however. When I add next test method I just start receiving below error message during build. Any ideas, resolutions?

Severity Code Description Project File Line Suppression State Error Type universe cannot resolve assembly: System.Configuration, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a. MyDriving.UnitTest.Core

Thanks for help