Try this:

Check that 8.0.xxx version is installed on your pc (add or remove programs, filter by sdk):

enter image description here

if it's not installed, install one from https://dotnet.microsoft.com/en-us/download/visual-studio-sdks

after that go to solution folder and create a global.json file with contents like (version should be of SDK you installed):

{
  "sdk": {
    "version": "8.0.313"
  }
}

more on global.json https://learn.microsoft.com/en-us/dotnet/core/tools/global-json