Abbot Jack might have gone to the Tower of Trades, which is overrun with monsters, so you should find him.
Open the Tower
At the Tower of Trades, do a bow at the door like the man at Alltrades Abbey taught you. The door will open.
Tower of Trades - L1
On L1, there are just monsters and a few barrels. Go up the right-hand stairs.
Tower of Trades - L2
Go south and east through a door. There are some blue treasure chests out here. Then go up the stairs in the center of the room.
Tower of Trades - L3
You can read the inscription up north. In the southeast corner, there is a chest with a mini medal inside. Make your way to the stairs in the northwest corner of the room.
Tower of Trades - L4
Go to the southwest corner of L4 to find a red chest. Then go to the north part of the room where there is another red chest. Finally, go to the southern area where there is a blue chest. Then go up the stairs.
Tower of Trades - L5
Go north to find a red chest. Go into the room to find a couple of barrels. You can go to the northwest corner of this room to find some jars to break, but that's all that there is. Go up the middle of the room and up the stairs.
Tower of Trades - L6
There's nothing interesting on this floor, so just go up the stairs in the middle of the room past the columns.
Tower of Trades - L7
You can read the inscription just north of the stairs if you wish. There is a red chest in the southwest corner and northeast corner. Go through the door to the north.
Master of Nu'un
Jack of Alltrades becomes Master of Nu'un. He attacks you. He can cast two spells per turn and usually casts an attack spell along with a defense spell. His attack spells often go haywire and do massive damage. When he is defeated he drops a lava lump.
Return to Alltrades Abbey. Now you can talk to Jack to change your vocation. There are various people around with missions for you, some of which will require you to change to a different vocation and level up in it.
When you are ready, you can move on to Porth Llaffan.