You cannot simply click "Get" on the current GarageBand page. You will see an error: "GarageBand requires macOS 13.0 or later."
If GarageBand does not appear in your purchase history, it means you have never downloaded it with that Apple ID. In this case, proceed to Method 2 or Method 3 below. Garageband Mac Os X El Capitan 10.11.6 Download