Hansi Flick did succeed in giving the best tactics to Barcelona players last season, but they also still need to be given a way to maintain their mentality when playing in the Champions League so that when they are in a losing position while competing in the Champions League, their mentality does not immediately drop. Because it is true as you said that building a steel mentality in each player is indeed not easy and even quite difficult even though the player is indeed very compatible with the tactics applied by his coach.
Not many Barcelona players have experience in the Champions League competition so it is natural that they do not look so strong yet, but slowly this will improve. Hasni Flick managed to change Barcelona to be better than the previous season and this is a great achievement. Next season we will see how Hansi Flick can return to providing better results or not because the competition in the Champions League will certainly be much more competitive and a number of other teams have also made team overhauls by appointing a number of new players