You seem to be one of the coaching/training staff in Barca going by your first statement.
If you think Bartra was terrible in two Bayern games, I suggest you watch the matches again. Do not just assume that more numbers of goal means its the fault of the most inexperienced defender. Our mid-field was not in form, Pique continued his off form and Dani was being, well, Dani. He covered the whole right part of the defense along with the middle. He didn't have his best game, but it was better than average and the best of our defense in the 1st leg.
Playing CB for Spain U21 is not a cakewalk. The difference between the young national teams are not that much because of lack of maturity. Again, I suggest you watch the matches.
There are valid reasons why Pep didn't choose him. during Pep's last 2 seasons, Puyol and Pique were mostly healthy and in form. Mascherano was the 3rd choice and even he got very little play time until Puyol got injured. And Bartra was still very young and Fontas was higher in the pecking order ahead of him. Last season was different - he was an established CB in the B team, Puyol was injured almost the whole season and even Mascherano got injured in the 2nd half. I'm not saying he should have been a regular starter - just that he deserved to play in more matches - especially against the weaker opponents in liga. And If you think Tito managed the youngsters very well, then explain the Montoya situation. The guy wasn't even chosen in the dead rubbers and 30% of his match time was as LB.