Translation:We are on the third floor, not the second.
jianzhiw, tercero seems to be one of those adjectives which drop the final "o" before a masculine noun. See http://elblogdegramatica.blogspot.com/2012/11/definicion-de-apocope-ejemplos-de.html
28.07.2108 "we are on the third floor not on the second" rejected. reported. But google translate gives "estamos en el tercer piso, no en el segundo". for the above phrase, and agrees with DL. that without the "en" "no el segundo" signifies simply "not the second" . Moving on...
In the current example, when counting (and expressing) masculine nouns we drop the O ending for only PRIMERO and TERCERO. So if we changed the number we'd say, "Estamos en el septimo piso. Vamos al tercero." The anomaly that you are touching on is actually regarding primer and tercer. None of the other numbers (as far as I know) are shortened this way. As to WHY -- I have no idea.
That's extremely unlikely, as Duolingo does not recognize commas, periods, or indeed spaces, so writing "We! areonthe ?thirdfloornotthesecond!" looks the same to the system as "We are on the third floor, not the second." I advise you to look more closely (next time) for another error.