"You do not have the required quest state" - Different Issue

There was a bug that we fixed in 0.9.12h where you could use party portals to skip the Vaal Ruins and the unleashing of the darkness in Act 2. This was only in Cruel difficulty and above.

Try going back to the Vaal Ruins Level 2 and opening the Ancient Seal. Since you've already killed the Vaal Oversoul, it won't actually make the world dark, but it should give you the required quest state to access the areas past the Vaal Ruins.