There may be no easy 'resolution' for running this from FossaPup, whose PPM uses official Ubuntu repos but is not 100% compatible with what these repos provide. In particular, if this Ubuntu package is written to use systemd, then you indeed have a problem since FossaPup does not support systemd as an init system. Whilst it would of course be possible to get the package working with other init system that would require a dev to specifically work on that and have any hardware involved so no easy solution I think.
Well... there is probably an 'easy' forum-oriented solution - use a forum-available distro that is 100% compatible with the Ubuntu repos the package comes from. On this forum, that would be FossaDog64 (which I presume does support using systemd).
Alternatively, if same package is available from Void repos then use a distro that is pretty much 100% compatible with Void repos, which is KLV-Airedale64; Void's version probably expects runit (rather than systemd) and KLV-Airedale uses runit.
In other words if you need a way to get this programmed then use a Dog and not a Pup (even VoidPup probably won't help since that doesn't use runit - or does it(?), I can't remember, and temporarily don't have VoidPup installed because I ran out of space on my old machine). Otherwise, you will indeed need to hope someone works out how to get the package running without systemd on a Pup (may be possible without having the hardware to test), which seems to be a struggle. Is it possible to 'program' the keyboard on one distro (e.g. a Dog) such that it can then be used on another (a Puppy)?