Why does iMovie export .mp4 that QuickTime won't play?

Where did you export the video to? Is it on a local folder in your Movies folder or on an external volume or on iCloud Drive?

You may get the error message for several reasons -

• when the export has not yet been complete. It can take several minutes to render a video when exporting from iMovie
• or when you saved the video on a volume with an incompatible file system format, for example a case-sensitive file system may be problematic.
• Or when QuickTime does not have the necessary permissions because the volume has been used for Time Machine backups.

It sounds as though you have the Quality set at "Best (ProRes)" which produces huge .mov files.

Use either "Low, Medium or High" for .mp4 movies you want to display.

Best (ProRes) is only for special occasions when you may want to re-edit your video at a later date.

What version of QuickTimePlayer are you using? On my Mac, running Mojave and iMovie 10.1.12, QuickTimePlayer version 10.5 plays all Mp4 movies exported by iMovie.

Try deleting preferences and then exporting again. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project. To delete preferences, open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects.  Share out your project again and see if the exported movie will play in QT.

If still no luck getting the exported movie to play in QT try doing an Edit/Select All, Edit/Copy, of the media in your project and pasting it into a newly created project. Then share out the movie and see if QuickTimePlayer will play it.

Failing that, try converting your troublesome video to H.264, Mp4/AAC with the free download, Handbrake. Do that even if the codec already is indicated to be Mp4/AAC. Re-rendering often cures issues.

You can get Handbrake here:

https://handbrake.fr/

A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to complete. Then save and import the converted clip into iMovie.

-- Rich

SORRY! I misread your question.