If GarageBand is not in your purchase history, you cannot find it on the El Capitan App Store.
This is the definitive final release of the "modern" GarageBand (v10) that supports OS X 10.11. garageband el capitan
We hope this article has been helpful in getting you started with GarageBand on El Capitan. Happy creating! If GarageBand is not in your purchase history,
"Purchase" (it’s free) GarageBand on that newer machine to link it to your account. Happy creating
If you’re running OS X El Capitan (10.11) on an older Mac, GarageBand remains a fully capable digital audio workstation (DAW) for music creation, podcasting, and audio editing. While not the latest version, the GarageBand release compatible with El Capitan (typically version 10.1.x) still offers a robust set of tools.
How can I install Garageband on El Capitan - Apple Community
: Users report that GarageBand 6.0.5 runs reliably on El Capitan, provided it is upgraded from a previous system rather than a clean install, as re-downloading legacy versions can be difficult.