"He writes a letter to his girlfriend."
Translation:Γράφει μία επιστολή στην φίλη του.
We are working on a second revision of the course and hope to make things more consistent there -- double accents where grammatically required and consistent use or non-use of final nu are two of the big things.
(As for the final nu, it doesn't help that the rules for that change occasionally and not everyone agrees with the current version of the rules, nor does everyone speak the way it's taught at school. But for educational software such as Duolingo it's probably best to teach the current official spelling, though we may accept others even if we don't show them during the teaching.)
What was your entire sentence?
Did you, perhaps, use the wrong gender article?
Remember that γράμμα is neuter and επιστολή is feminine, so "a letter" would be ένα γράμμα but μία επιστολή -- if you write μία γράμμα or ένα επιστολή (or worse: ένα επιστολί), that is simply wrong.
And don't forget the accents: they are part of the spelling in Greek. γραμμα, επιστολη are not correctly spelled.
It's not that it's not needed when it's singular, it's rather whether the meaning changes if you omit it. Since ένας/μία/ένα is also the numeral one, using it can change the meaning - but not always.
In any case, this sentence should be fine without the indefinite article. If it was rejected and you come across it again, please report it using the specific button to create a notification for the course contributors.
Γράφει μία επιστολή στην φίλη του. = He writes a letter to his girlfriend. or He writes one letter to his girlfriend. (Arguably the first makes a lot more sense, that's why this is the translation you see.)
Γράφει μία επιστολή στην φίλη του κάθε εβδομάδα = He writes one letter to his girlfriend per week. (Wouldn't "a letter per week" work too? Would it matter if both were correct?)
Γράφει επιστολή στην φίλη του (κάθε εβδομάδα) = He writes a letter to his girlfriend. i.e. a letter, not a book, not doodling on a post-it. :) Quantity is not important, it the form of communication that's he's chosen that we're focusing on.