"I am a builder and my dad is a writer."
Translation:Я строитель, а мой папа — писатель.
Duolingo doesn't take punctuation into account when marking answers right or wrong. The reason you got this one wrong is because you wrote "мои" instead of "мой". "Мои" is plural 'my' and "мой" is masculine singular 'my'. Your answer, though only off by a letter, was incorrect. The dash is not compulsory here.