#1 Trying to play their way out of Liverpool's Gegenpress
At the start of the match, Liverpool were pressing very high and causing the away side problems but still, Barcelona were trying to play out from the back. Even after Liverpool got their first goal from a Barcelona mistake, the away side didn't learn their lesson and continued their stubborn style.
That tactic cost them dearly in the second half as another Barcelona mistake caused by their playing out from the back policy. Alexander-Arnold was allowed space to cross in and he found Wijnaldum who smacked it in to make it 2-0.
Barcelona were only inviting pressure by trying to play out their way from the Gegenpressing as it is famously known. Jurgen Klopp bases his football on never giving up and chasing every lost cause and Barcelona would have been benefited if they tried to clear their lines more often than playing their way out.