I read Flick's interview after the game and it seems he believes that his team is not clinical enough[1]. I also saw some tweets blaming Lewandowski because he lost possession too many times and failed to convert good goalscoring chances into goals. Whether it was his fault or not, I do agree that the team needs to improve their chemistry on the pitch so they can create more chances. I guess that happens if you don't have a good backup.

[1] https://barcauniversal.com/hansi-flicks-scathing-assessment-after-barcelona-lose-1-2-to-las-palmas-this-is-our-problem/