GarageBand for Mac (v10.0): Unable to complete in-app purchase
When trying to make an in-app purchase from GarageBand for Mac (v10.0) you might be unable to complete the purchase. This can happen if you haven't accepted the app from the Mac App Store.
To resolve the issue:
Open the Mac App Store.
Sign in using the Apple ID you used to set up the computer.
Click the Purchases tab.
Click the Accept button next to GarageBand.
You should now be able to make in-app purchases.