From what I understand once the Majin Buu arc ended Uub became a pupil of Goku and was to become Z-fighter. But in Super so far I haven't seen any mention of him at all.
Does he not exist in Super yet? Or have I missed something crucial?
The final tournament takes place AFTER the events of Dragon Ball Super. DB Super picks up immediately after the defeat of Majin Buu (Kid form).
Thus characters such as Uub, Pan and Bulla are yet to be shown in DB Super.
Dragon Ball Timeline: All Events This might be an interesting read.
Age 774, September 7 : Shenron erases everyone's memories of Majin Buu.
Dragon Ball Super
Age 778: Gohan and Videl are married, and Videl becomes pregnant with her and Gohan's daughter, Pan.
Age 778, August 18: The events of Dragon Ball Z: Battle of Gods and Battle of Gods Saga take place.
Age 779: The events of Dragon Ball Z: Resurrection ‘F’ and Resurrection ‘F’ Saga take place.
Pan is born.
Age 780: The events of the Champa Saga take place. The God of Destruction of the sixth universe, Champa wants to have a tournament between universe seven for their Earth.
The events of Potaufeu Saga take place.
The events of the Future Trunks Saga take place.
Age 784, May 7 : The 28th World Martial Arts Tournament takes place. Goku leaves with Uub to train him
Does he not exist in Super yet?
He does. But he's younger than at the end of Dragon Ball Z. In Dragon Ball Super, Dende mentions to Goku that there is a kid in a small village with a great potential that he should train, and that the boy is a reincarnation of the evil Majin Buu.
dragon ball super did not start at the Uub part they just started right after they beat kid buu so its like they just cut off the Uub part and that never happend because after battle of the gods they got the idea of revival of f then got the idea of dragon ball super because at first they were gonna make a season about goku training Uub to fight him