.mov files can be tricky because they are a bucket that can contain different codecs for audio and video. So, .mov files can vary from one another.
Open your .mov file in the free download, Handbrake, and convert it to Mp4/AAC that Handbrake will do by default.
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 open the file in QuickTimePlayer or import the converted clip into iMovie.
-- Rich