Barcelona did not play consistently last season and conceded a lot, but it all depends on whether Barcelona can score more than they concede. At the beginning of the season there were no problems with goals, but gradually Barcelona stopped scoring and in the match against Las Palmas Barcelona had chances, but except for Rafinha nobody could score.
They did better in early of this season with good defense and attack but they began to have more problems in recent matches. These latest bad performances destroy good advantage they had only few weeks ago. They are becoming more vulnerable and more easily have conceded goals, that can destroy this season. Hansi Flick must work with his young players, keep they on the ground despite of how many applauses they got, and also have to work with medical team to take care of old players very carefully. These young squad, if lack of support from several old key players, will very easily collapse.
Barcelona lost with more than 20 shots on goal and it is very bad for any team, if they have 20 shots on goal, they can score only once. We can see that Barcelona started to decline in November and even the return of Yamal and Torres did not help
It happens in a bad day with any club, you are a big club, dominate the match, but you fail to score enough goals to beat your opponent, and in counter attacks, they beat you. It's a very bad day for Barcelona but they have to work hard to prevent similar matches, reduce probability of having similar bad matches in future.
Barcelona's advantage can be forgotten, because they are separated from Atletico by only 2 points, and Real have 2 games in reserve and if Real win 2 times, they will be ahead of Barcelona by 2 points, so there is no advantage for Barcelona anymore.
I could agree with you and say that any team has such games, when the team creates 10-20 chances, but does not score, but in the game against Las Palmas struck more the physical condition of Barcelona players. It seems that they are tired as if they are playing the end of the season and there are 1-2 matches left before the end of the season. It is not clear how Flick will deal with the tiredness of the players, because his style of play depends very much on the physical condition of the players and now that the team is not as fresh and full of energy as at the beginning of the season, you can see that Barcelona players look tired, but Flick's bench is not so long that he can immediately strengthen the game.