Translation:The girl has neither a mother nor a father.
Pardon me in advance for neither English nor Russian are my first language,i believe "нет" here,is part of bigger phrase considering how "у девочки нет ..." as a whole would imply "the girl doesn't have ..." and to make it more clear "у девочки есть" means "the girl has" hence proving that while "есть" means something like "is" and is probably considered positive,"нет" on the other hand is the exact opposite and means something like "isn't";so "нет" being part of the possession related half of the sentence is in fact a negative word
so, suddenly, mom and dad are not accepted any more although it has ALWAYS seem to have been, so far, but suddenly, i need to type mother and father? oh maaan. the "nope, wrong" sound annoys me when the "mistake" does not apear as one to me. in russian, i observe many cases of "was right all the time and is no more, all of a sudden"! i notice by myself what i then approvingly read in numerous comments - again and again. sad detail of this otherwise aaaawesome app! i hope the it crowd of duolingo can soon turn this issue off and not on again ;)