Barcelona don't have the luxury of buying as many players as they want. Unlike Real Madrid, who can buy any player they want, Barca's financial problems mean they have a spending limit. Therefore, Barca need to build a very balanced squad. Therefore, for Hansi Flick to be successful, he will need to keep players like Vitor Roque, Eric Garcia and he will need to know how to get the most out of every player at the club. For now, Hansi Flick's job is not easy and Xavi said before leaving Barca that Flick must be ready to suffer.
When Real Madrid was struggling to win Champions League in the years Barcelona were taking it subsequently, Barcelona was busy greasing Messi and some of their strong hold footballers with with huge some of money. Anytime Messi want to leave, they make him change his mind with more funds and debt and when the money is becoming debt and weren't coming again, he left and the debt and deficit are there to date and they can't flex like the way Real Madrid are doing today.
Another challenge is that they haven't been able to win major leagues to be able to attract sales, winning trophy means utilizing the opportunity to make money from fans because they will do anything to but tickets, but jersey and other VIP events by the club but right now, some fans will rather use the money to eat than go for Barcelona event when their is nothing special.