If you’d like, I can:
For El Capitan 10.11.6, the compatible versions are typically GarageBand '11 (version 6.0.5) or an early release of GarageBand 10 The "New Computer" Workaround 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. If you’d like, I can: For El Capitan 10
: The App Store should recognize your OS and offer the last compatible version for 10.11.6. Downloading an older version of Garageband on Macbook If you’d like