I looked into downloading Garage Band on the internet and the download is free but the current version of Garage band being offered is 10.3.4 and the download info says you must have Mac OS 10.13.6 or later to use this version. Now, I thought that Garage Band came bundled as part of the operating system download but apparently not in my case as I don't have it but I would like to start using it. This version of the operating system was installed onto my iMac at the Genius Bar/ Apple Store about 18 months ago.
I have an iMac and I'm using Mac OS Sierra 10.12.6.