Turn Undead was working on a Dybbuk who has a corpse
In the rules for the possession of the Dybbuk expressly states:
The Dybbuk is effectively the obsessed creature now. His type becomes undead […]
If it has a corpse, it is an undead creature, and no other description or rule gives it immunity to undead or other effects specifically aimed at undead creatures. If it were immune to such things, the rule that says its type will become undead would have no meaning at all!
Outside a corpse, the dybbuk is a fiend; In a corpse it is undead, with all the mechanical consequences that result. If the Dybbuk does not want to be susceptible to undead busting effects, he should clear the body.