Your Mac needs Monterey to run the current iMovie version. So, upgrade your Catalina to Monterey and you will be able to download current iMovie 10. However, as thesurreyfriends said, you should already have iMovie 10 on your Mac, in the Applications folder.
If your computer is too old to upgrade to Monterey, and if you have or ever have had, iMovie 10 on your Mac, then you can use the following procedure to obtain the highest iMovie 10 version that your Mac will run:
To update/redownload iMovie 10, drag your iMovie 10 app from the Applications folder into the trash, but do not empty the trash. (You can skip that step if you had iMovie 10 on your computer before, but deleted it.) 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 iMovie 10. Redownload/update iMovie from there. If not successful, drag your present iMovie app out of the trash (if you moved it there) and return it to the Applications folder.
-- Rich