An Impossible Foe

Stleech hit the nail on the head here.

Even though the Empowered Traits will automatically unlock now once you fill your original 35 Traits, for the Artifact Challenge Appearances from the Mage Tower you do still need those quests from 7.2 completed to attempt the challenge itself.

(The questlines are shared across different specs and classes, so when you are checking out the guides there, it'll help to pop open your browsers search feature and type in your spec to help point you in the right direction)