"This version of OSX is not supported on this platform." when booting 2012 Mac Mini with SSD drive from previous system
Each Mac has a minimum build number that has the necessary drivers to run that Mac. Sometimes a build will run tens of Macs and some builds are very narrowly scoped to handle all the existing models and that one new release until those drivers can be incorporated into the next numbered release of the OS.


Mac OS X versions (builds) for computers


What this means for you, is you'll need to try booting to the recovery HD on that SSD and hope the version of recovery is new enough to work to download he proper installer and let you proceed with the build for that Mac. If not, you'll need to place the drive that came with that Mini back in (or into an external USB enclosure that costs about $5 to $10 and hold the option key) to select that external drive.

At that point, you can re-run the installer or migrate the data from the SSD to the external drive. Then you can wipe the SSD clean using recovery and migrate back without having to reinstall or lose any data or settings. 

There are many paths to getting back in business - but knowing the minimum builds will let you check things next time to run the updates before you move the drive.