PSP X6 was released about August last year - X7 is the current version. It's fully compliant with Win 8.1 - but for whatever reason not server.
I'm not entirely sure if this helps but do you need to ensure that the server has the Application server role and also the feature to enable desktop graphics.
Oops thought you were referring to an old Paintshop pro from 2000 or so.
I know this doesn't answer your question, but why do you need Paintshop Pro X6? Could what you're trying to do be done with more up-to-date (and likely just as good) free tools like Paint.NET or GIMP?