You can use your Time Machine backup, if you have one, to restore your older iMovie 10 iteration.
However, iMovie 10.1.13 (current version) does work on Mojave 10.14.6. So you can try updating to that if you have not done so already.
Another thing that you can try is to reinstall iMovie. To do that, drag your present iMovie app from the Applications folder to the trash, but don't empty the trash. Then sign in at the online app store and click on your name at the bottom of the app store side bar. That will open your account. Look at your list of purchases. iMovie should be there and you can redownload it. You will need to be running Mojave 10.14.6 to run it. If for some reason the re-download is not availble, you can drag your iMovie app out of the trash and replace it in the Applications folder.
-- Rich