Why do I get a "No test adapters are referenced by this solution" error message?

As the message implies, you need to install some NuGet packages, one for the testing framework and one for the visual studio test runner. If you are upgrading an old solution using MSTest, you first need to remove the reference to Microsoft.VisualStudio.QualityTools.UnitTestFramework.

Then you need to open the Package Manager Console (under Tools-> Nuget Package Manager -> Package Manager Console). Select your unit test project(s) and run the following commands:

Install-Package MSTest.TestFramework
Install-Package MSTest.TestAdapter

If you are using a different test framework, such as XUnit or NUnit, you have to install either XUnit and xunit.runner.visualstudio, or NUnit and NUnit3TestAdapter

I was able to see the tests in Test Explorer but they were being ignored.

I found and deleted a Local.testsettings file in the solution folder and it fixed the problem (I had earlier updated the nuget packages as the other answer suggests but that was not enough in my case).

I created a brand new project in VisualStudio 2017 and was getting the same error message until I installed xunit.runner.visualstudio NuGet package. Follow these instructions on the Xunit getting started webpage (https://xunit.github.io/docs/getting-started/netfx/visual-studio)