Modern versions of GarageBand (10.x and later) require newer macOS versions (like Big Sur or Monterey). When you attempt to download GarageBand from the App Store on El Capitan, you will likely encounter an error message stating: "We could not complete your purchase. GarageBand requires macOS 11 or later."
Click to copy the software back to your active Applications folder. System Performance Tips for GarageBand on El Capitan Garageband Mac Os X El Capitan 10.11.6 Download