Cannot complete first dungeon goal

For every single dungeon, the first goal is to “Defeat this Dungeon on Difficulty 1” (or maybe it’s an I, I cannot tell). What is “Difficulty 1”? I’ve ran the Dragonguard Depths 83 times now, on various difficulties including Easy, Medium, and Hard. I am lv 45 so that’s all available to me. What am I missing here? Is this goal not accomplishable until after lv 50?

Correct - DIfficulty I is the first difficulty after hard, available upon reaching level 50.