"My grandfather's name is Raj, not Aamir."
Translation:मेरे नाना का नाम आमिर नहीं राज है।
It should be if you said "आमिर नहीं" instead of "नहीं आमिर." दादा is a grandfather from your father's side of the family and नाना is a grandfather from your mother's side of the family. We don't distinguish that in English.
That's what answered (मेरे नाना का नाम है आमिर नहीं), but it marked me as wrong.
मेरे नाना का नाम राज है, आमिर नहीं should be accepted since the structure of the sentence is such. The positive phrase with Raj comes first, and the negative section with Aamir later.