Ben Foster said on his podcast fairly recently that if he was a manager signing a goalkeeper, the first thing he'd want to see is a compilation of his mistakes. His reasoning was he would be able to tell if it was a case of the goalkeeper having the right reasoning but maybe not executing it well enough or just not being technically good enough.
With Onana it's clearly the latter. I've never seen a goalkeeper make so many inexcusable errors.
He has to move on this summer, it should have been a number one priority. The team will never get anywhere with a player so hopelessly out of their depth.