If you're experiencing issues with mExtension and your subscription plan is active, try the below:
Please close Final Cut Pro. Now, please go to your main “Movies” folder, press “CMD+SHIFT+. (period)” - this will reveal hidden folders. Now, you'll see.mExtension gray folder - delete it, and re-install mExtension via the .pkg file available here: mExtension.
Restart your Mac, and check if the problem is resolved.