Baez misplayed two of the first three balls hit to him (naturally, the first three batters hit the ball to center).
According to the Cubs announcers, the first ball was hit high over the center field wall, and Baez went all the way to the wall allowing it to bounce back towards the infield, rather than playing the ball off the wall. The second ball was hit to him without problem. The third one he played a circular route, finally having to try a running, knee high catch, which he missed for an error.
It is possible that he will improve with experience.