They can certainly book IC 552 in Cluj (or any other domestic train), but the problem may be that they consider the change time too short and therefore don't want to sell you the second reservation in advance.
In any case, the Romanian planner doesn't give this connection and I'd also consider it too short (and too early), given Romanian timekeeping.
IRN 12767 leaves Cluj at 23:04. CFR don't really really propose any convenient connections and lets you wait between midnight and 3:30 in Dej.
To reach IC 552, CFR propose to take IRN 1838 from Cluj at 21:25, which arrives in Suceava at 4:03.
But what is wrong with the direct night train leaving Cluj at 19:40 and arriving in Bucharest at 6:17? There's still availability in all comfort classes.
Also keep in mind that, when you change trains after midnight, you'll need another travel day if you're using a flexipass. Or a regular ticket of course.