Can you provide a screenshot of this, please? (Upload it somewhere and paste the URL here.)
So that I can attach it to a bug report.
"this" and "the correct answer" are rather vague, I'm afraid, and I can't see what kind of exercise you got (multiple-choice, translate, type what you hear, fill in the blank, tapping, ....) as the system automatically generates a number of exercises from one and the same sentence.
nein "no" is the opposite of ja "yes" -- it's an answer to a question.
It does not mean "not", nor is it used in phrases such as "there are no people here".
I did the same thing as everyone else. Typed in correct answer, came out as incorrect. Turns out, that I'm so good at translating, I wrote what I heard in English.