You are not student...
Why it is not correct answer?
In English you either say "a student" or "the student", but you MUST use an article.
is it correct if I omit "sen" so the sentence would be just " öğrenci değilsin" ?
Yes, you can say that. Here, "sen" is used let's say if there are many students in an area, and you want to point out the person that is not a student.
Çok teşekkür ederim
How would one say, "You are not the student."?