Actually this is not only for Lewandowski but for the whole team even though in the first match Barcelona controlled the game with very large possession of the ball but the chances for them can be counted because of the solid midfield and Bayern defense in cutting attacks.
On the other hand, Barcelona always makes the mistake of allowing Bayern's players to attack back so that defeat is unavoidable.
Allowing Bayern players to strike back is ridiculous for Barcelona. Because maybe Barcelona underestimated the Bayern players when attacking back so that Barcelona players didn't make a fuss when it happened. But they (Barcelona players) all feel sorry when they lose just because they made a small mistake like you said.
This is a very annoying thing for Barcelona and I believe Barcelona can still win in the next game.