Andre Onana is a goalkeeper who is still very young, doesn't have much experience, and can be said to be immature because he still has points of gaps and weaknesses when the opponent manages to get a score from a kick in the middle of the field and break through Andre Onana's defense. Hopefully, in next season's match, Andre Onana will perform better and focus so that it makes it difficult for opponents to score goals.
Very young? He is 27 years old, it isn't the age for a youngster anymore.
I think he is experienced enough, he already played in 2 top clubs before joining Man United. He was a goalkeeper of Ajax and Inter Milan. If he still couldn't perform with his best, it is because he is in adaptation with Man United tactic. He also still doesn't know well the nature (game style) of Man United players. These may be some logic reasons why he is rather hard to show his best.
Onana has made a couple of errors in pre season, overall it wasn't costly because it happened in friendly games and it gives him the opportunity to correct his mistakes, so it would not turn costly and happen in a competitive game.
Yes, he was lucky he did it in pre season matches. He must be aware that he must evaluate his performance. It won't be acceptable if he does it again in EPL matches. I hope he can get enough lessons for the mistakes, so we won't see the same mistakes in the competitive matches. Man United mustn't expect to see more unnecessary errors from Onana.