Which system version is running on your Mac? And which GarageBand version has been the last version you used?
You can only purchase the current version of GarageBand 10 from the AppStore, if your Mac is running at least macOS 10.13.6 High Sierra.
But you may want to check your Purchase History at the AppStore. Is GarageBAnd in your list of previous purchases? If you have been using GarageBand '11, version 6.0.5, it may still be showing in your last of purchases (the fourth tab "Purchased") at the AppStore. In that case you could try to reinstall the older iLife version by downloading it from your previous purchases. Look for the older version with the guitar as the icon, not the newer version with the guitar and the stomp box: