SpecRun.Runner NuGet package that contains the Visual Studio Test Explorer adapter is a solution-level package (registered in the
.nuget\packages.config file of the solution). In some situations, NuGet package restore on build does not restore solution-level packages.
To fix this, open the NuGet console or the NuGet references dialog and click on the restore packages button. You may need to restart Visual Studio after restoring the packages.