"You are late!"
Translation:Du er sen!
Two reasons. First, because you included the word for, which means 'too'. Duo isn't asking for "you are too late" this time, only for "you are late."
Second, because the addition of the final -t makes it either an adverb, or the neuter form of the adjective. As fveldig has said, the masculine/feminine form sen is used for humans. (For what it's worth, I also just typed in 'Du er sent' and it was accepted, but with the note that 'sent' contained a spelling error.)