"I do not have a grandfather."
Translation:У меня нет дедушки.
"У меня нет деда" should be accepted. Duo developers do not like short Russian words and expressions. They probably think the longer the better.
У меня нет деда isn't still accepted. Even though this is an absolutely right and normal answer. I reported.
Why is the accusative of дедушка дедушки instead of дедушку? I thought this kind of masculines were declined on their "femenine" form. Plus дедушка ends in а.
Right, when you use negative the genitive case must be used. Declinations are hell for a speaker of a language where declinations don't even exist...