I know there is another question with exact the same problem, but I went trough all those answers, and none helped me. :( (This was the question.)
I just created a new A
In my case, I had to open the .csproj file and add the reference by hand, like this (Microsoft.Extensions.Identity.Stores.dll was missing):
..\..\..\..\Program Files\dotnet\sdk\NuGetFallbackFolder\microsoft.extensions.identity.stores\2.0.1\lib\netstandard2.0\Microsoft.Extensions.Identity.Stores.dll