"Are you writing a book?"
"You" is both singular and plural. So, if you used "Γράφεις" or "Γράφετε" would be correct, if your sentence was rejected it was due to an other error. Since we're unable to see any of what the learners write we have no way of knowing what that was. Did you use the drop down hints.? They clearly give..."Did you write a book.'
Did you report it as "My sentence should be accepted."? It would be helpful if you had.
According to the incubator, it is correct. But since we can't see your sentence we have no way of knowing if there was some error that caused it to be rejected.
Please make a report when your sentence is rejected and you don't see the reason.
TIPS TO MAKE LEARNING EASIER + HOW TO REPORT A PROBLEM
And check out the Greek Forum here with more links.