Apple no longer supports El Capitan. The current version of GarageBand requires macOS Ventura or later. To get a compatible version, you must download the last compatible release . Title: How to Download GarageBand for Mac OS X El Capitan (10.11.6) Last updated: [Current Date]
If you are still running and need to install GarageBand, you cannot simply download the latest version from the App Store—your Mac won't recognize it. Garageband Mac Os X El Capitan 10.11.6 Download
If you ever update your OS, you must delete this old GarageBand and install the new 300+ MB version (which requires 2GB+ of sounds). Did this work for you? Let me know in the comments. Apple no longer supports El Capitan