If you ever had iMovie 10 on your Mac, but perhaps deleted it or lost it in some
way, you can still download the latest iMovie version that you present operating
system can run. To do that, sign in to the app store and open your
purchases folder by clicking on your name at the bottom of the app store side
bar. You will see your purchases displayed, including (hopefully) iMovie. Redownload/update
iMovie from there.
If you still have an earlier iMovie 10 on your Mac, then drag it from the Applications folder into
the trash, but do not empty the trash. Then do the above procedure. You can drag it out of
the trash if you need to.
If the above does not work, you probably will not be able to get iMovie 10. You would need
to update to at least Mojave 10.14.6 to get current iMovie, and, unfortunately, your computer
cannot update to Mojave. (Some websites purport to offer an iMovie 10 download, but such
websites often are unsafe and can cause malware to be transferred to your computer.)
Even if you never had iMovie 10 on your computer, you might try the above procedure anyway, on the outside chance.
-- Rich