If your El Capitan machine refuses to cooperate, you can use a newer Mac (borrowed or secondary) to download the older installer to your Apple ID’s purchase history.
Downloading GarageBand for OS X El Capitan (10.11.6) is challenging because the Mac App Store Garageband Mac Os X El Capitan 10.11.6 Download
In some instances, Apple hosts standalone legacy installers on its official support servers. While Apple has phased out many direct DMG installers for GarageBand 10, older versions (such as GarageBand 6.0.5, also known as GarageBand '11) can sometimes be updated using packages found on the Apple Support Downloads page. Go to the official Apple Support Downloads website. Search for "GarageBand Update." Look for . If your El Capitan machine refuses to cooperate,