If GarageBand is not in your history, sign in with your Apple ID on a newer Mac that is compatible with the current version. "Purchase" (download) GarageBand there to link it to your account. Once it is in your purchase history, return to your El Capitan Mac, go to the Purchased tab, and you should now be able to download the older compatible version. Compatible Versions for El Capitan
“Not tonight, you don’t,” Leo whispered to the screen. Garageband Mac Os X El Capitan 10.11.6 Download
How can I install Garageband on El Capitan - Apple Community The Last Song on El Capitan Issue 1: