welcome to .NET development!
By default, Visual Studio will add references to the most common namespaces/classes to your project when the project is created. TestTools.UnitTesting is not one of those namespaces, so you'll have to add a reference to the file where it lives, which is in Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll
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Check out this thread for how to add references (I think your question duplicates it):