Microsoft Visual Studio 2019: The project file cannot be opened. Unable to locate the .NET SDK

Try to edit envrionment Variables.

Right click on This PC -> Properties -> Advanced System Settings -> Advanced -> Environment Variables -> System Variables

Add [install path] C:\Program Files\dotnet\ to the variable path.

Restart visual studio.

If it does not solve this problem, you can refer to this answer.

I found that the path C:\Program Files\dotnet was already in my path but it was ordered below the C:\Program Files (x86)\dotnet entry. By simply moving C:\Program Files\dotnet above C:\Program Files (x86)\dotnet in the list and restarting Visual Studio cleared up the problem.