Please try what Keith suggested. When you want to download from the AppStore there is a big difference between the main tab of the App Store, where you purchase an app (even fee apps) and the tab with your previous purchases.
- On the main entry page, the first tab, you can only purchase apps for the most recent system versions.
- From the list of previous purchases you can download again older versions you previously purchased . Just make sure you sign into the App Store with the same AppleID you used when you first purchased the app on an older system version.
It will depend on the system version where you are are finding your previous purchases at the App Store. Until macOS 10.14 it has been the fourth tab of the App Store.
Which system version is running on your Mac from 2012? In 2012, before Apple introduced the Mac App Store, you have probably received Garageband '11 on the installer media coming with a new Mac. Use your old installers to reinstall the GarageBand version that came with your Mac, the one you own. The iLife version of GarageBand is so old, that the developer certificate of the installers has expired, and Apple has removed all updaters from the downloads page. If you never upgraded to GarageBand 10 at the AppStore and missed the opportunity to get it, when the version you need has been the current version, it is not possible to purchase GarageBand 10 now.
