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The error that you are encountering is a "Missing Media” type alert which indicates an outdated version of FCPX.
Please update Final Cut Pro (for free - via App Store) and everything should work just fine.
If you don't see any available updates, 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 after that you will be able to update FCPX to the latest version.