If your computer specs allow, upgrade your operating system to version 10.15.6 or later. Otherwise you will not be able to obtain the current version of iMovie.
If not possible to upgrade to the above version, try the following procedure to get the highest iMovie 10 version that your computer operating system can run. (It requires that you either now have, or in the past had, an older version of iMovie 10 on your Mac.)
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. (NOTE: For El Capitan and older, click on the Purchases tab at the top of the app store screen and then sign in. Click on Purchases again to reveal your purchases.) 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