The problem occurs after upgrading the Test Studio Visual Studio plugin to 2015.3.1202 version. In Visual Studio your test projects will no longer build.
As of version 2015.3.1202 Test Studio has migrated to .NET 4.5. If your test project was created in .NET 4.0 the above issue will occur after upgrading the Test Studio Visual Studio plugin to version 2015.3.1202 and above.
You have to upgrade your Visual Studio projects to .NET Framework 4.5 as the target framework from the properties of the Test Project.
1. Right click on the test project and click Properties.
2. Under the Application tab select .NET Framework 4.5 as the target framework.
3. Confirm the target framework change.