Downloading a specific older version of GarageBand for OS X El Capitan (10.11.6) can be tricky because the App Store usually only shows the newest version, which requires a much newer macOS. To get a verified, safe copy, use the following methods: Verified App Store Method
By following this report, users can confidently download and verify GarageBand on Mac OS X El Capitan 10.11.6. garageband mac os x el capitan 10116 download extra verified
To verify the download for your El Capitan machine, you need access to a second Mac running macOS High Sierra or newer (or borrow a friend's Mac briefly). Downloading a specific older version of GarageBand for
If you absolutely cannot get GarageBand via the App Store: garageband mac os x el capitan 10116 download extra verified
and follow the Purchases tab steps above. Alternative: iLife '11 Physical Media