Here's another thing that worked for me. I use ASP.NET v8.0 but my project file from uses v6.0. Here's a step by step procedure on how to resolve the error:

1. Open your .csproj file.
2. Change the version from the version in the file to the version you are using.
3. Save and close the file.
4. Restart visual studio and the application. It will work for sure