Transcode issues in FCPX Extension
If your MXF file or other media is failing to transcode, install Pro Video Formats from Apple
Jan 23, 2022
If you are trying to import an MXF file or another media type and receive a transcode error message from the Simon Says macOS app / FCPX Extension, download and install Pro Video Formats from Apple:
https://support.apple.com/kb/DL2050?locale=en_US
(Apple may update this; feel free to search for a newer version)