If you're running Mac OS X El Capitan (10.11.6), you can download and install GarageBand from the Mac App Store. Here are the system requirements:
GarageBand is a popular digital audio workstation (DAW) developed by Apple Inc. It is widely used for music production, podcasting, and live performances. This report provides a step-by-step guide on how to download and install GarageBand on Mac OS X El Capitan 10.11.6.
You must "purchase" the app first. If you try to download it from the main featured page, the App Store will tell you your OS is too old. To bypass this:
Go to System Preferences > Security & Privacy . Under the "General" tab, click "Open Anyway." 2. Missing Core Content
If you'd like to move forward with your music project, I can help you: for this version Troubleshoot a specific error code you're seeing Suggest lightweight VSTs (plugins) that work on El Capitan
However, because El Capitan was the last major version of OS X before the transition to macOS Sierra, it sits in a unique "compatibility zone." You can still get a fully functional version of GarageBand, but you cannot simply hit the "Download" button on the main App Store page.
using your "Purchased" history. Because the current version of GarageBand requires much newer software (macOS 14.4 or later), it will not show up in standard search results for older systems. Apple Support Community Recommended Download Methods Via App Store "Purchased" Tab