First download the tune onto your iDevice, then hit “import”
iDevice owners looking to import a song into the mobile version of Apple’s GarageBand may be dismayed to see that iTunes Match songs may appear to be unavailable. However, that’s not entirely the case.GarageBand iOS cannot import songs that are not stored locally on the iPhone or iPad. As such, users must download the song first, then import it.
To download a song, open the Music app on your iOS device, navigate over to the Albums view and click the Download from iCloud button. Your download should begin now. As soon as the song finishes downloading, you’ll be able to import it into GarageBand.
Apple offers additional useful tips on importing songs from the Music app for iPhone and iPad via these support documents.
▪ iPad: Import a song from the Music app.
▪ iPhone/iPod touch: Import a song from the Music app.