I used 'du er sent' and it works. I believe the issue is the 'for', maybe try 'du er altfor sen/sent' (which wouldn't translate to exactly what duo is asking and so would probably be marked wrong anyway? Not sure)
In the audio exercise, they say: "Du er sent". But in the writing exercise, when you write that, it says that you have a typo and that the correct answer is : "Du er sen". Those should be the same thing... I'm not sure I understand.