Can you provide a screenshot of the exercise you were asked to do?
If it was a translation exercise, I assume you were given the sentence "you write" -- that should let you know that Sie schreibt. (= she writes) is not a possible translation and that you have to choose Sie schreiben. (= you write).
If you are obviously bad at German, they might be a little surprised at the use of du but will put it down to your being a foreigner and not knowing how to speak German properly.
If you appear to speak reasonably decent German and address a stranger above the age of 25 or so as du, they may well get offended.
If you address a policeman as du, you may even get a fine.
The subject and the verb form do not match.
Verb forms for du almost always end in -st, so it would be du schreibst.
schreibt with -t would be the verb form for er, sie, es or for ihr, so ihr schreibt is another possible translation for "you write", when you are speaking to several people rather than just one.
Because you can't leave out the subject pronoun in German -- it has to be Ihr schreibt. and not just Schreibt., when you are speaking to several people whom you know well.
Or Du schreibst. if you are speaking to one person whom you know well, or Sie schreiben. if you are speaking to one or more people formally.
- Sie schreiben.
- Du schreibst.
- Ihr schreibt.
All are correct translations of "you write", depending on how many people you are speaking to and how well you know them.
Sie schreiben. can also mean "They write." -- at the beginning of a sentence, it's impossible to tell the difference between sie (they) and Sie (you).
How do we know whether "you" is singular or plural, or how well we supposedly know this or these people?
Generally, you can't -- and so all three options for "you" will be accepted.
The verb ending has to match the pronoun you choose, of course -- something like du schreibt or ihr schreibst will not be accepted, not because of the pronoun, but because the pronoun and the verb do not match.