A known working version is "Garage Band 10.2.0-2017," which is confirmed to work on OS X 10.11.6. This version was originally obtained from the App Store and has been archived for preservation purposes. This version is available for free download and borrowing through the Internet Archive.
The sound library didn't finish downloading.
2. How to Download GarageBand for El Capitan (The "Last Compatible Version" Method) Garageband Mac Os X El Capitan 10.11.6 Download
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A prompt will appear stating: "The current version requires macOS X.X or later, but you can download the last compatible version." Click to confirm. A known working version is "Garage Band 10
While running older hardware requires a few extra steps, getting a functional Digital Audio Workstation (DAW) like GarageBand running on El Capitan is entirely possible with a little patience and the right download history.
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. The sound library didn't finish downloading
Before downloading GarageBand, make sure your Mac meets the system requirements: