For users who have never downloaded GarageBand, a workaround exists but is cumbersome:
If you have ever downloaded GarageBand in the past using your current Apple ID, you can download the last compatible version directly from Apple's servers. Garageband Mac Os X El Capitan 10.11.6 Download
Here is the step-by-step guide to getting GarageBand on El Capitan. For users who have never downloaded GarageBand, a
The most reliable way to get an older version is if you have "purchased" it (even for free) on your Apple ID in the past Check Your Purchase History Mac App Store on your El Capitan machine Apple Support Community Click on the tab or your account profile at the bottom of the sidebar Apple Support Community Find GarageBand in the list and click the Cloud icon A prompt should appear asking if you want to "Download the last compatible version" for your OS . Confirm this to begin the download. The "Borrow a Newer Mac" Trick Confirm this to begin the download