If he wants playing time, Rashford needs to work hard because if his performances aren't consistent, it will be difficult to earn Flick's trust. Barcelona has a number of quality players in that position, and it won't be easy for Rashford to get regular playing time. Rashford has great talent, but his ego is too big, which means not all coaches like his style and style of play. If this can be addressed, I'm sure Rashford will return to his best form at Barcelona over time with consistent training.
Rashford simply needs to play according to the coach's instructions when he's deployed in his preferred position, as the Barcelona coach will then assess Rashford's level of cooperation with his teammates. Barcelona is currently striving to make significant improvements in order to have a better season and free itself from financial difficulties, so Barcelona should be able to win several major competitions next season. And Rashford's arrival at Barcelona could at least add a little strength that could have a positive effect on the team as a whole.
It's not certain that Rashford will get playing time; it won't be easy, with Rapinha and Lewandowski also in the squad, so Rashford will have to compete for a starting spot.
Barcelona's reason for signing Rashford was to fill a void, similar to last season when injuries caused a dip in performance. If he wants to develop further, Rashford will have to prove himself in the limited opportunities he's been given.