Note this error happened in the unpacking phase, not the configuration phase. So I think it is unlikely dpkg has even looked at swapspace' postinst at this point.
Also note the previous line is "Processing triggers for systemd".
I note that swapspace doesn't have a preinst script.
So I would speculate the problem is in the systemd trigger.
(this being complicated so far has nothing to do with systemd, but rather the design of dpkg)
Looking at /var/lib/dpkg/info/systemd.postinst I would speculate the trigger being called is after /etc/init.d is updated, it calls "systemctl daemon-reload" via a wrapper that checks /run/systemd/system exists first.
The only time I have seen this happen myself is when systemd was already broken, because the kernel was too old and didn't have the required features.
Possibly nothing wrong with the kernel here, however I have to wonder if systemd was already broken for some reason. Maybe it failed to start up correctly because something else was broken.