"The girl does not wear a green skirt."
Translation:Das Mädchen trägt keinen grünen Rock.
No. Don't use 'nicht' and 'ein' together in a sentence. (Although on rare occassions this works) 'Kein' is virtually always the better choice. EDIT: to be more precise: your sentence now is grammatically correct but it means something different 'The girl has several green skirts and she wears them all except one'
Now the sentance wasn't translated. I do that myself quite often where I read the sentence, record it in my head wrong then when I write, I write what I want the sentence to be rather than what it is. If it said THE skirt and DER were in your translation it would work. If skirt were plural, it also might work as it would say that the girl does not wear green skirts, presumably because she does not like the color.
"Das Mädchen trägt keinen grün Rock"
Since keinen already speaks of the gender/case of the object, does green also have to agree with the gender/case?