Otherwise, the error that you encountered is a “Missing Media” type alert, which typically indicates an outdated version of Final Cut Pro or macOS. Please update the app (for free, via App Store), or system via settings. Make sure to create necessary backups!
As next step, make sure that mInstaller is updated - you can check that in the app settings. After the update, uninstall and install your plugins again.
If you see the Missing Media screen on the newest version of Final Cut Pro though, it might indicate a problem with the plugin's registration in the system. Make sure that your mInstaller is updated to the newest version. Once that's done, uninstall the plugin and install it again anew, without using the “repair” option. Now check if all is back to normal.
If the issue persists, try repairing the plugin registration by following these steps:
Close all running instances of Motion/FCP
Switch to Finder and go to your Applications > MotionVFX > Plugins folder,
Right-click mUtility, and select “Open” (do NOT double-click it - macOS detects that as a different type of action). “Nothing” will happen, as it works only in the background!
Do the same for mOSC and mOSC 2 (if you can see this one)
Start FCP and see if the problem is solved
If the above doesn't work, please delete the mUtility, mOSC, and mOSC 2, and empty the trash. Now, re-install one of the plugins via the mInstaller if you're trying to use a perpetual product, or re-install mExtension in the case of subscriptions, and let us know if this worked.