Does this sentence mean, "You live longer than them"?
Yes, they are translated the same in Romanian. Now that you said it, yours sounds more correct in english than the one given in the exercise.
If i use meaning "tu" (not "voi") why is it mistake?
In English, 'you live more than them' would be used to indicate that you have a better or more exciting life than them or that you make better use of your lifetime than them.
It would not mean you had lived longer.