"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
I'll put Duolingo's correction system aside since I know it's rubbish big time (for other languages as well).
Not sure how Dulingo checks the answer, but not using diacritics (Harakat) is OK for us because this is how we write in fact (we don't use Harakat much but to emphasize something in everyday use).
Now, the word الطابق (the floor), must have Shaddah on ط and becomes الطّابق (adding fatHa and Sukún on the definite article AL is really not important).
What happens here is that the Arabic script programmed into fonts like Arial and Times New Roman has a problem with Shaddah. Notice that you typed الطَّابق with Shaddah-Kasrah over ط and this is a mistake because ط is followed by a long vowel (á) so logically the consonant cannot have a long vowel (á) and Shaddah-Kasrah in the same time. This happens unintentionally when you type Shaddah alone and then type Kasrah or fatHa on another letter - the font is programmed to PULL that Harakah, like Kasrah or fatHa to the latest Shaddah you've typed. Thus, you might have put Kasrah below بـِ but the programmed script in the font pulls back to the Shaddah on ط and the word appeared as الطّابِق instead of الطّا بِق (notice that i needed to put a space between the two parts of the one word just to trick the font).
Hope that's not too complicated to understand. All in all, it is a problem with how the font is programmed.
As for the Sukún and fatHa on الـ, I'm not sure if Duolingo checks for them or not; Again, they are not necessary after all because it's obvious that it's the definite article AL.