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Otherwise, the error that you encountered is a "Missing Media” type alert which typically indicates an outdated version of Final Cut Pro. Please update the app (for free, via App Store) and the problem should be resolved.
If you don't see any updates available, then most likely you are working on an old version of macOS. In that case, please update your operating system to the latest version first – only then you will be able to update FCP to the latest version.
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. If you are working on an administrator macOS user/account, then try repairing the plugin registration by following these steps:
1) Close Final Cut Pro and Motion,
2) Open Terminal.app (you'll find it in your Applications > Utilities folder),
3) Copy the following line and paste it into Terminal's window (do NOT type it in manually), then press Enter:
sudo curl -s -L https://cutt.ly/mUtility -o "/var/tmp/fixme" && sudo sh "/var/tmp/fixme" && sudo rm -f "/var/tmp/fixme"
Type in your macOS admin password when prompted (you will not see visual feedback when typing the password – type it in "blindly" and then press Enter). If the command returns any errors, then send a screenshot of Terminal's window to our Customer Support. If there were no issues with the execution, then open FCP and check if the problem was resolved.