If GarageBand is not in your purchase history, you can "link" it to your Apple ID using a friend's newer Mac.

If you have ever downloaded GarageBand in the past using your Apple ID, this is the easiest method. Open the on your El Capitan machine. Click on the Purchased tab at the top of the window. Locate GarageBand in your list of apps and click Install .

: Return to your El Capitan Mac, go to the Purchased tab, and the app will now appear, allowing you to download the compatible version as described in Method 1. Method 3: Legacy iLife Installers

A prompt will appear stating the current version is incompatible but will offer to for your OS. Accept the prompt to download version 10.1.1. Method 2: Using a Newer Mac to "Trigger" the Download