#1 Lack of creativity from Espanyol

It was not so surprising to see Barcelona with plenty of chances, as they have a lot of playmakers to create things. So when you confront this, you either stop them from doing that, or do the same.
Barca cannot be stopped from finding gaps, and the opposition must be prepared enough to create as much as they can. While Espanyol at least tried to stop barca by pressing, they never looked to create.
Even after having some genuine possession spells, the players moved the ball sideways and backwards only, forcing them to look for opportunities through crosses and set pieces. With Gerrard Pique and Clement Lenglet in that form, the team could afford only one set piece goal, which was eventually nullified by VAR.
All these demanded the white and blue striped men find gaps and play some crisp football to find the net. Failing to even attempt such plays, the scene looked like the case of lack of preparation.