My iMovie file couldn't be opened, file isn't compatible with QuickTime player

That does sound a little strange. QuickTimePlayer should recognize an Mp4 file.

Try deleting preferences and then exporting the project again. Then see if it can be read by QuickTimePlayer. 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.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

Also, try putting your media into a newly created project and see if the exported video will play in QuickTimePlayer.

If still no luck, try re-rendering the movie with the free download Handbrake. 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