Match vs Alaves wasn't good. He was disappointed that after a 0-3 in CL the team didn't have a more fitting response in terms of pride and mentality. There's issues in the team that are tied to Flick also, not only the players. The super advanced pressing he expects can't be central to absolutely everything.
There's some issues that can be improved by Flick also. Move the ball more and be more patient in build-up, don't expect Casado to offer similar passing quality to Pedri for example. When you have Pedri out and Casado in, you have to tweak the way you play and play more cautious. Or else you find yourself giving the ball away too much in the wrong places, which is what Casado did. That goes against Flick's football, which is build on directness and pressing. The pressing lowers the risk of his direct style. Flick is not really a possession manager at heart. Barca and German styles can be similar in places, but the Barca philosophy is also built on patience, control, and more elaborate and long passing sequences.
The relation is between the risk you take with your passing and the quality of pressing. You can take more risk when the press is better. And the risk you take with your passing depends on how good the off the ball movement is from more offensive players and obviously the individual quality of the passer. An easier pass for Pedri is not as easy for Casado.
All 3 are suffering and generate more bad turnovers that lead to even weak opponents having many clear chances vs Barca. Some players don't move well or often enough to provide passing options, the individual quality of the passer moving from Pedri to Casado took a massive hit, and the press when the ball was lost was also subpar.
You can tackle some of these things, but Flick doesn't like compromise.