With Revit 2021 you should not need to download anything to use Dynamo. With the installation of Revit, Dynamo should be installed and ready to work. So something has happened. Something that comes to mind?
-Have you tired to install another Dynamo version
-Addins that might influence the working? Try to deactivte them
-Have you looked for updates for Revit 2021?
-Have you tried a repair/reinstall of Revit 2021?
Louis

Please mention Revit version, especially when uploading Revit files.