Barca were 4-2 ahead in the closing minutes before Atletico fought back, so leaving Flick frustrated.
Flick said afterwards, "I warned against giving away any chances. I'm disappointed with the result because, seeing how we played after conceding two goals... we created a lot of chances. In the last few minutes we conceded too much.
"We had chances to win. We will have to talk about the goals we conceded. We have to correct our mistakes. The team is very young, there is a lot of room for improvement.
"I'm frustrated but happy with the game. But conceding goals can't happen. The opponent played very well. We played against a good, strong team. We dominated for 80 minutes. I'm satisfied. I think the fans enjoyed it. We didn't throw in the towel and we'll be able to win there.
"We have to work hard, we have a very young team. They scored four goals against us and it's too much for us.
"I think it's a cup match and everyone likes this championship. Atlético has a fantastic team. In the last few minutes we didn't defend well. I couldn't believe it with the fourth goal and we have to talk about it. The important thing is that the team has confidence. We have to be positive."
He also said, "We have to give them the opportunity to make mistakes. The reaction is the most positive thing. I'm satisfied with this, but also frustrated with the result and the last 10 minutes. Many players have given their best. Balde, Cubarsí, Frenkie... they are very good and they are giving the best version they can offer. And Lamine. He is very good and we have to continue like this.
"If you train hard, you show it in the game. If you train at 70 percent, you don't defend at 100 percent. The quality of the sessions is very good. I'm very happy about many things. I'm 95 percent satisfied, but four goals are too much."