.NET Windows Forms is not available for Linux; It's a .NET wrapper over Windows user interface libraries, such as User32 and GDI+.
According to the Windows Forms documentations:
Windows Forms is a UI framework for building Windows desktop apps.
If you are interested in building a cross-platform app, you may want to take a look at .NET MAUI (which is not available for Linux yet, but available on a few other platforms):
.NET Multi-platform App UI (.NET MAUI) lets you build native apps using a .NET cross-platform UI toolkit that targets the mobile and desktop form factors on Android, iOS, macOS, Windows, and Tizen.