Onana actually made some mistakes and contributed to some losses of Manchester United but it's unfair to say everything is his responsibility. At beginning, it is wrong to think that a single player, a goalkeeper can improve a defense system that has many holes. Manchester United's defensive strength comes from many positions, goalkeeper, defenders and midfielders or if seriously we can also say players at their front line too.
Even though Onana made mistakes in several matches, I agree with you that Erik Ten Hak's focus should also be to improve the defense line, so that Onana can communicate well with Man United's other lower players. I tried to look back at the beginning of David De Gea's arrival at Old Trafford and the same thing happened to him, but slowly David De Gea's performance improved until he became Man United's mainstay goalkeeper for a long time before Onana's arrival. So let's see how Onana can learn from the mistakes he made so that he can slowly become David De Gea's ideal replacement.
The spotlight must not only be given to the goalkeeper, but the defender is also responsible for several mistakes that occurred which resulted in defeat for Man United. If the defender plays well then the goalkeeper will also perform well because the last line of defense is the defender before facing the goalkeeper. Onana did make a mistake due to a blunder, but when the defender plays well, maybe he can play much better than now.