When you say completed the requirements, did you find all the stuff to gather or did you also "craft" it at the Requisition table? (Any req table will do for the advanced class items.)
Once you choose a spec, the trainers go hang out by the NPC companion that has that spec, at least I've seen that a couple of times anyway. Solas is also a Rift Mage, so go check by him. I've seen a few cases where something can advance the state of Skyhold before it's supposed to, so the trainer might have moved there. (Just in case you didn't know, Solas is in the little round room off the main hall, on the right side as you enter it next to Varric.)
