"Since when is she writing?"
Translation:Seit wann schreibt sie?
The element governed by “seit” must be dative and in this case it is “wann” (which, being an adverb, has no dative form). “Sie” is simply the subject of the clause, it doesn't matter whether it comes before or after “seit”, its role is what determines case.
Just to clarify the point further: pronouns in English also change forms after prepositions (e.g. “after me”, “before her”), but they don't when they just come after them but are not their argument (e.g. “after the meal I felt full”, “before then she was happy”).
Duo generally marks misspellings as wrong only when the resulting word is actually a different word. I don't know which part of the sentence you are referring to, but the order of “i” and “e” in German makes a lot of difference: they are pronounced differently, they can completely change the meaning of a word and sometimes they are the only indicator of whether a verb is present or past tense, for example: “schreibst” = “you write”, “schriebst” = “you wrote”.
Duolingo can't possibly know your intentions, it only sees what you type, and if you mistype in such a way that a new sentence with a different meaning is formed, then it corrects you.