Cant switch difficulty?

I am not sure is this a bug or just I miss something?

My lvl 37 ranger just completed all A1-A3 quests but can't go to next level.

I'm sure they are all done and there is no quest items in my inventory.

I tried many ways but it's not working,

what should I do?

Try to kill Piety again, she should drop the key to the Scepter of God, which you then can enter from the Baracks.