"I am a writer."

Translation: Я письменник.

[JohanBoonstra]

I answered 'я - письменник'. It is correct. But is the '-' necessary?

[Vinnfred]

Dash is used between a pronoun and a noun only for emphasis. It's obligatory to use it between a noun-subject and a noun-predicate

[greg.mort]

You don't really need the "-" there. Duolingo doesn't check for capitalizations and punctuation marks anyway.