This is perhaps a permission issue, typically happens when SketchUp does not installed properly.
- Uninstall that plugin.
- Close SketchUp.
- Make sure no other instance of SketchUp is running. (Perhaps restart your computer)
- Find your downloaded installer file - (or download a fresh copy from here: SketchUp Pro 2021) - right click on that installer file and chose Run as administrator.
- When prompted chose Repair. Let the repair finishes.
- Restart your computer.
- Start SketchUp and install the plugin. (make sure it is the latest version)
- Close SketchUp.
- Start SketchUp.