"Scrivete un libro."
The difference is simply between singular and plural. "You", (tu scrivi) in the singular, and "all of you" (voi scrivete) in the plural. They both translate to "you." Grammatically this is the second person singular and the second person plural. Hope this helps :)
(Sorry caludio, I just don't think imperatives are an issue in this question)
You're right, but my point is that "Scrivete un libro" as a self standing phrase has a meaning ONLY if used as imperative. But yes, duolingo is often weird :)
"Tu scrivi" is "You write", but it's not imperative. In italian (like in any other language) the imperative doesn't use the pronoun "Scrivi!" or "Scrivete!". So, "scrivete un libro" is "Do write a book", while "You write a book" is just the present tense "Tu scrivi un libro".