"There is a grandfather and a grandmother on the second floor."
Translation:في ٱلْطّابِق اَلْثّاني جَدّ وَجَدّة.
Well, you can add it in fact (not sure if Duolingo would accept the answer though)
but you would have to reverse the order of the sentence in Arabic: هناك جد وجدة في الطابق الثاني
It can be done also with the help of commas and keeping the current order as above: هناك، في الطابق الثاني، جد وجدة
I would personally understand any of these sentences but not sure about Duolingo