It’s not you. There have been other reports of the same thing. It has to do with some registry entries not getting updated. Make sure that when you installed SketchUp 2021 you right clicked on the installer and used Run as administrator from the context menu. If you didn’t, you can run the installer again using the run as administrator option and then Repair.
If that doesn’t sort it out, you can manually edit the registry if you want. See: