Translation:In my grandfather's room there is an antique piece of furniture.
As long as we really understand the Italian we can always debate all the possible translations.
Nella stanza di mio nonno = In the room of my grandfather
c'è un mobile antico = there is a furniture antique
In my grandfather's room there is an antique piece of furniture / a piece of antique furniture.
Both variants are approved.
As said, there shouldn't be anything wrong with either of joesci's suggestions, therefore I think both are pretty correct. But, only his second suggestion on my displayed thread here - 'In the room of my grandfather there is an antique piece of furniture' - is being accepted [by Duolingo]. I don't know why the former is being marked wrong, so I'd obviously be inclined to questioning why so, also.
Duo's answer for this, while perfectly good standard English gramatically, is not something any of the people i know would ever say. One antique piece of furniture would be identified more specifically. e.g. In my grandfather's room there is an antique desk. A generic answer of this nature would be used for multiple pieces rather than listing each item. e.g. In my grandfather's room there are several pieces of antique furniture. If he has antique desk, chair and armoire for example. Just saying. Not expecting it will change any time soon.
I think the important thing is that we really understand. Things can always be expressed in different ways, both in English and in Italian In order to learn I find it useful to translate both in a literal way and, especially when that is not how it would be phrased in the other language, change it to something that works and is as close as possible.
c'è mobile antico = there is furniture antique ~ there is a piece of antique furniture