I recently set up a new Mac mini server for a client. macOS High Sierra was released 3 days ago, however the server shipped with macOS Sierra and I thought it prudent to stick with tried and tested 10.12 rather than launch into the wild, blue yonder with a 3-day old 10.13 setup.
This was all well and good until it came to downloading Server.app from the App Store.
I logged in with the client’s general-purpose Apple ID and went to purchase Server.app from the App Store. With the new release of macOS, Server.app had just been bumped to v5.4 which only supports 10.13. The App Store would not let me purchase Server.app as I was running 10.12.
There are two ways around this problem. They both result in the machine with 10.12 being able to download the previous version of Server.app that is compatible.
You can either sign in to a machine running 10.13 with the general Apple ID and purchase (but not download) the software, or if you have purchased Server.app previously on another Apple ID, sign in with this one.
Once you sign in to the App Store on the 10.12 machine with an account that has already purchased Server.app, you can click on the Install button in the Purchases history and then you’ll be informed that this version won’t work with your version of macOS, do you want to download a previous version.
Answer yes to this, the download starts and you can happily set up Server.app on an older version of macOS.