It is way to early to say he can't play. He has 21 starts across college and the pros. He just has so little experience. I agree that his pocket awarness is awful but he has spent so little time there is isn't surprising.
The thing is I think the only way it improves is live reps. And I am not sure he is getting them in Dallas any more than he was in San Fran or in Buffalo.
The only way you improve Trey Lance is live reps. I am not saying he is or will become Josh Allen but he needs to chance to make all his mistakes and to have his bad days and see what NFL defenses throw at you the way Josh did in 2018. When you get someone who is raw, inexperienced and has mechanical issues they don't learn to get better watching from the side.