نوبت دوم اضافه نمودن ماگريشن

سلام طبق آموزش شما پيش ميرم. مايگرشن ساخته شد. ديتابيس ساخته شد. و اونجايي که بايد user را تغيير ميدادم. ديگه ماگريشن جديد اد نميشه. لطفا راهنمايي فرماييد.

Add-Migration mig_changeUser
The specified framework version '2.1' could not be parsed
The specified framework 'Microsoft.NETCore.App', version '2.1' was not found.
  - Check application dependencies and target a framework version installed at:
      C:\Program Files\dotnet\
  - Installing .NET Core prerequisites might help resolve this problem:
      http://go.microsoft.com/fwlink/?LinkID=798306&clcid=0x409
  - The .NET Core framework and SDK can be installed from:
      https://aka.ms/dotnet-download
  - The following versions are installed:
      1.0.5 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
      1.1.2 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
      2.0.5 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
      2.0.7 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
      2.0.9 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
      2.1.2 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
      2.1.5 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]