Could an orphan/rogue planet have a moon? Is there some reason why an orphan or rogue planet wouldn't have a moon? Let's say it started out in a normal system and then just got flung out by an unstable orbit.
Could an orphan planet have a moon?

Yes.

s there some reason why an orphan or rogue planet wouldn't have a moon?

Yes, this answer lists some possibilities.
What you didn't ask was:

Is there some reason why no orphan nor rogue planet could ever have a moon?

and based on the previous two, No.
Any non-contact gravitational acceleration that could pull a planet away from its star has some chance of accelerating its moons sufficiently to stay with the planet.
If the perturber were a few times the Moon's orbit farther from the planet, staying with the planet might even be likely.
Looking at this obliquely from a delta-v perspective, Jupiter orbits the Sun at 13.1 km/s, and its inner Galilean moon Io orbits Jupiter at 17.3 km/s.
Without doing the math, I think it's possible that you could give Jupiter a 13.1 km/s kick impulsively in an optimal direction and Io would stay bound to it, but now in a much more elliptical orbit.