If your project is a slideshow and you export it as a video to your desktop it will be in the M4v format. That format is an Apple format that works most places, but may not work on a PC. To be safe, you should convert the M4v video to to Mp4 that is almost a universally compatible format. You can make the conversion with the free download, Handbrake, that will convert to Mp4 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 import the converted clip into iMovie.
NOTE: I am using Photos 4, so if you are using a later Photos version that exports slideshows in Mp4, then you are good to go without the necessity of converting.
-- Rich