Do not have required quest state

Did you die during the fight? It might be possible you were not in the area (resurrecting) when the boss died to an ongoing skill (like a damage over time or a skill with a duration like firestorm). It could also be some bug where the game didn't register you going through the opening into act 3. I think that even if you kill him, if you exit without going through that opening, you won't be able to move on to act 3.

However it happened, looks like it'll probably fix itself if you kill him again. You don't need to have the apex if the game thinks you've already been there. Just click the altar to start the fight.