Ensure your macOS is fully updated to 10.11.6 (build 15G31 or 15G22010). Go to Apple Menu → About This Mac → Software Update. After updating to 10.11.6, restart and attempt the download again.
OS X El Capitan 10.11.6 , the most compatible and updated version of GarageBand you can typically run is GarageBand '11 (version 6.0.5) or an early version of GarageBand 10 (such as 10.1.x) Apple Discussions How to Download GarageBand for El Capitan Because the Apple App Store garageband mac os x el capitan 10116 download extra updated
garageband mac os x el capitan 10116 download extra updated Ensure your macOS is fully updated to 10
Apple stopped supporting OS X El Capitan (10.11.6) with newer GarageBand versions (10.2 and later require macOS Sierra or higher). The last fully compatible release is (sometimes referred to as version 10.1.2 or 10.1.4 depending on sub-updates, but 10.1.6 is the final). OS X El Capitan 10