Abbot Jack might have gone to the Tower of Trades, which is overrun with monsters, so you should find him.
Open the Tower
If you haven't already, you need to assign Bow to your Party Tricks. Press X to open the menu, then choose Misc., then Assign Party Tricks. Move the cursor to a slot and press A, then go to page 2 and choose Bow. Then you can hold the B button and press the button that you assigned the party trick to. Go up to the door at the Tower of Trades, and do a Bow at the door. It will open.
Tower of Trades - L1
On L1, there are just monsters and a few barrels. Go to the northeast corner and go up the stairs.
Tower of Trades - L2
Go south and east through a door. There are some blue treasure chests out here. Then go clockwise until you can go northeast to the stairs in the center of the room. Go up those stairs.
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 that contains iron ore. Then go to the north part of the room where there is another red chest, which contains 1200 gold coins. Finally, go to the southern area where there is a blue chest. Then go up the stairs in the southeast corner.
Tower of Trades - L5
Go north to find a red chest, which contains iron gauntlets. The northeast room has 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 that contains a mini medal, and a red chest in the northeast corner that contains magic water. Go through the door to the north. Talk to the person you find in the center of the room.
Master of Nu'Un
Jack of Alltrades becomes Master of Nu'Un. He has 796 HP. 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. If he has cast Buff on himself, attack him with magic if possible.
Don't use Woosh or Zam spells on him. He's strong against those.
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.
You may find it helpful to temporarily change everyone's vocations to Thief and level up until you have 10 Skill Points, which you can use to learn the Half-inch skill. This skill can be used even if you switch to a different vocation, and it allows you to attempt to steal items from enemies, which makes it easier to get alchemy ingredients.
When you are ready, you can exit Alltrades Abbey and go to the south part of this island to reach Porth Llaffan.