How can I install Garageband on El Capitan - Apple Community
A prompt should appear stating that the current version is incompatible, but offering to "Download the last compatible version" for your OS. Apple Support Community Workaround for New Users Garageband Mac Os X El Capitan 10.11.6 Download
Here is the definitive guide to getting GarageBand up and running on 10.11.6. How can I install Garageband on El Capitan
How to Download GarageBand for Mac OS X El Capitan (10.11.6) Garageband Mac Os X El Capitan 10.11.6 Download
If you owned your Mac when El Capitan was the current operating system, or if you have older physical installation media, you can bypass the App Store entirely.