Motion for Mojave 10.14.6

You can’t buy an older version directly. You need to buy the current version on a mac with Catalina or Big Sur using YOUR Apple id.

Then back at your mac with Mojave, open the App Store, click your account at the bottom left then click the Download button to get Motion. You will be warned that the current version doesn’t work on your OS and asked if you want the most recent compatible version.

If you really want Motion, there are two things you can do: either ask a friend to let you buy using their mac (with YOUR Apple id) and then delete it; or install Catalina or Big Sur on an external drive and use it to do tge purchase. The latter option has the advantage of letting you try the new OS without risking any incompatibility.