On OS X El Capitan 10.11.6, the App Store will likely download (or similar builds from the 10.1.x branch).
Finding a working download link for GarageBand on Mac OS X El Capitan (10.11.6) can be challenging. Apple’s official App Store typically only distributes the latest version of GarageBand, which requires a much newer macOS.
El Capitan uses Apple's Gatekeeper security feature, which will often block unauthorized installers from running unless you manually bypass system security settings.