I agree with Joe and Kate. Both are correct but the meaning is different. To 'lack concentration' means that someone always struggles to concentrate or they aren't very good concentrating. To 'lose concentration' means that previously you were concentrating on something but you got distracted or you stopped concentrating for some other reason.

Wow, in this instance you could make a strong argument for either option, for different reasons. But I agree with Joe that lack is the slightly better option because it seems they are attempting to list two different character traits, instead of saying that dreamy people tend to lose (their) concentration.

lack Although the difference is very subtle. "Lack(s) concentration" is more of a character trait. "Lose(s) concrentration" is more of a behavior or tendency. To me, "lack" sounds better.