Lamine Yamal gave his side the lead from the penalty spot just before half-time but Barcelona were unable to capitalise on their lead.
Fran Perez scored the equaliser in the 67th minute and the game ended 1-1, leaving manager Flick furious with his players.
According to The Athletic, the German went as a far as locking his players in the away dressing room, frustrated over the perceived drop in standards.
He is understood to have given the players the same message he later told the media, only with a few more expletives.
Speaking to the press after the game, he said: "The most important thing is that, when the transfer window is closed, everyone who stays is 100 per cent committed to this club.
“No egos, because egos kill success. Last season, we played as one unit. We have to be like this again now."