He was quite error prone when he lost his place in the team under Lampard at the back end of 2019. But that was nearly 2 years ago. His performances for Milan, from what I've seen, have been several notches better than what he showed under Lampard, despite still having the odd lapse in concentration still.
But then again, Rudiger and Christensen looked terrible too until Tuchel got his hands on them, so I can understand Duffer frustration there.
Tomori's ceiling is high, and he should have made the Euro's squad on the strength of his loan performance alone, so the only way is up for him.