unfortunately, I cannot tell how to fix your issue from this information. Since it happens in a new project, I suspect there might either be a problem with your .NET installation (maybe try upgrading it to a newer version), or with the version of Godot you downloaded (make sure it is the mono version). If the problem persists, you may contact me here so we can try to do some deeper troubleshooting.
I am happy to hear that you were able to solve your issue. The warning you get now should not be a problem, and only appear when you open the project. If it annoys you, you can change all instances of "EditorPlugin.GetEditorInterface()" with "EditorInterface". The reason for the obsolete code is backward compatibility with Godot 4.0.