Like the others, the sentence is corrects as "You write" using the formal you "Sie". The twist here is that Duo is including this example in the strengthen/practice sessions before the user reaches the lesson titled "Formal You".
Think of it as a free preview of things to come? FWIW...I find myself learning these "gotcha!" examples almost directly because they are confusing at first and so require my attention to read and figure them out.
I understand that "sie" is she OR they OR formal you, depending on context. But "schreiben"? Isn't that the form used for "they"? When using the "formal YOU" word, does it follow that "formal you" is the same as "they"? Or am I just trying to figure out something I haven't been taught yet?
Non German here. But I have atleast mastered the pronunciations. You have to understand the parts of mouth that are involved in German language. It's pretty close to snoring sound (kh) but where the uvula vibrates instead of blocking the tonsil. Pronounce (sh)-->Pronounce (kh) using uvula --> Pronounce (ei) --> Pronounce(ben)
Do you have a screenshot of Duo rejecting "they are writing " as the answer to a translation exercise? If so, please share it with us -- upload it to a website somewhere such as imgur and then post the URL of the image in a comment here.
Otherwise, I will assume that you either (a) had a listening exercise ("type what you hear") and should have, well, "typed what you heard" (i.e. in German), or (b) made a mistake of some kind.