"Mayısta kızlar çok güzel."
Translation:The girls are very beautiful in May.
"the girls in may are very beautiful" is correct English even if duolingo soesn't recognise this.
I would disagree that this is a good English sentence. "The girls in May" suggests that these are a specific category of girls, like "the girls in the castle," or "the girls in Istanbul" for example. This would be a limiting definition of a specific group. What is being said in the original sentence is that girls are particularly good-looking in May, but their identity is not confined to that month. In other words, I think "in May" works as an adverbial phrase here, not an adjectival one, and thus should go either before the girls or after beautiful....
"mayıs" is May and the locative suffix (in/at/on) is "-da" which changes into "-ta" after the consonants "ç f h k p s ş t" . Hence "mayısta" which translates as "in May"