Comment 7 for bug 1563048

@mike-petersen-web - yes, in my case, it's definitely a dependency issue. I installed 16.04 server and then added Kubuntu Desktop. Whilst that installed sddm and its service, sddm didn't have any dependency on xserver-xorg-core, directly or indirectly (which it really should have, as it's an X display manager and runs /usr/bin/X :) ).