What steps are you doing to import it? What happens when you try?
iMovie does import HEVC. If it's not, try restarting your computer and opening iMovie while holding down the Option and Command keys and selecting 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. Now see if you can import your HEVC video.
You can also try importing to the Photos app and accessing the video from the Photos item in the iMovie sidebar. Your Photos library would need to be designated as the system library in Photos preferences.
Or, you can use the Image Capture app on your Mac to import the video to your desktop, and from there drag it into iMovie.
To cover the bases, make sure that you have enough free space for the import.
-- Rich