How is he average defensively? Because his defensive percentiles when it comes to tackles and interceptions on FBref said so?
Your position on the field is not an error. "Error prone" means exactly that - prone to errors. It has nothing to do with where you are on the pitch, so if he meant that he isn't positionally aware, then he should say that, and not error prone.
If you pass the ball to an opponent, that is an error, even if it doesn't result in a chance. He is one of the best passers in Serie A, so obviously I'm going to focus on his strengths and what we've been lacking in midfield. You should think of him like a deep lying playmaker first and foremost, and not a defensive beast, even if he looks like a defensive madman like Vidal. Him being decent defensively and at tracking runners is a bonus, but it's mainly Casemiro's job to do this, while Amrabat should be a focal point of possession and passing.
I don't even know why you claim he is bad at tracking runners, but it indicates you haven't watched him much. Besides, when you say he is mostly a sidegrade on McTominay, you pretty much lose all your credibility with this post.