Need iMovie 10

Not necessarily. You may have had iMovie 10 on your Mac when you bought it but accidentally deleted it, or lost it for some reason. Then you could retrieve it with the procedure that I mentioned.

Since you apparently never had iMovie 10 on your Mac, you can try the procedure described in this thread below that has worked for others in your situation:

iMovie for High Sierra 10.13.6 ???? - Apple Community

Particularly, read the post by spectre_ghost about half way through the thread.

-- Rich

If you ever had iMovie 10 on your Mac, even if you deleted it, you can try this procedure: You can redownload iMovie by signing into the app store and going to your list of purchases from your account. You can download it from there.

— Rich