Translation:You do not sit.
What is the difference between do not & don't that makes one right & the other wrong?
Wherever "do not" is accepted, "don't" should be accepted automatically by Duolingo and vice versa.
"You don't sit" is accepted for me when I try it out "behind the scenes".
If it didn't work for you in an exercise, I'm not sure why that might have been.
If this happens again, a screenshot can be helpful - upload it somewhere and post a link to it in the sentence discussion.