

Plug a 16GB (minimum size) USB thumb drive into your Mac. Now you are ready to create a bootable USB startup disk containing the Yosemite MacOS installer. It will install an app named “Install OS X Yosemite” in your Applications folder. It just installs the installation application so that you can run the installer. Your web browser will download the Yosemite installer as a disk image named InstallOS.dmg or InstallMacOSX.dmg. This next step does not actually install the MacOS. Scroll down to the Yosemite link and click on it.

The App Store will automatically check what MacOS you are running, and download an older, compatible version.

I made the mistake of doing a clean install of MacOS 10.13.6, which is High Sierra. I've seen a lot of posts with people having similar issues installing GarageBand on their older Macs.
