Migration from .NET Framework to .NET 5

Hello everyone, I am performing a migration of my solution from .NET Framework to .NET 5, all the projects in my solution are upgraded now, however as expected now I am running on hundreds of errors, primarily on missing dependencies.
One of those is regarding Model Class:
Severity    Code    Description    Project    File    Line    Suppression State
Error    CS0012    The type 'Control' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=xxxxxxxxxxxxx'.  

As well as one on Environment Class:
Severity    Code    Description    Project    File    Line    Suppression State
Error    CS1061    'Environment' does not contain a definition for 'Refresh' and no accessible extension method 'Refresh' accepting a first argument of type 'Environment' could be found (are you missing a using directive or an assembly reference?) 

So any ideas to solve them?