GarageBand

There’s no good means for a new user to download older versions of Garageband or other Apple apps (for macOS, iOS, or iPadOS), past what is in the Apple app store. What’s in the app store is what is available. This if you haven’t previously downloaded it on macOS when the macOS version and the app of interest was more current, and can check in the previous purchases listing.

If you have downloaded Garageband previously, then you’re either offered or are already running the newest version for your macOS version.

Your macOS version (OS X 10.11 El Capitan) is too old for any of the recent versions of the common apps.

Which iMac model and year? (See 🍎 > About This Mac...)

I’m assuming a macOS upgrade is not available for,your iMac, but it can be possible.

How to upgrade to macOS High Sierra - Apple Support

How to upgrade to macOS Mojave - Apple Support

How to upgrade to macOS Catalina - Apple Support (requires all apps, and all printer and scanner drivers, to be 64-bit.)

Have a backup before any upgrade.

You can only download it if your IOS is latest. To verify this, go to apps store, then type garage band. If it won’t show you need to update your IOS. Then try searching it again.

https://www.apple.com/hk/en/ios/garageband/