Hi,
Hmmm. That is strange, for sure. iMovie videos ordinarily are compatible with both QuickTimePlayer and You Tube. Does the incompatibility issue arise with all exported iMovie projects, or just this particular one? Try exporting to your desktop and playing a different project. Upload it to You Tube by dragging it into You Tube's upload window.
Also, you can try converting the exported video with the free download, Handbrake. Handbrake will convert/re-render the video by default to Mp4/AAC. Do the conversion even if the movie already is in Mp4/AAC format.
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