"We have a girl."
Translation:Εμείς έχουμε ένα κορίτσι.
Έχουμε ένα κορίτσι is perfectly correct
It is indeed. And it's one of the accepted translations.
so why is it signalled as wrong?
The most common cause is that you made a typo and wrote something else than you thought you had written.
If you have a screenshot of the question and your answer, please upload it to a website somewhere (e.g. imgur) and post the URL to the image here -- then we can see exactly what you wrote and point out the error.
Otherwise I'm afraid it will forever remain a mystery.
(Also, didn't you report "my translation should be accepted"? I see no reports for this sentence from today.)
See here for the best and most effective way to Report a sentence that was not accepted. This way we can see your full sentence and explain why it was not accepted, or we can edit our incubator if it was correct and should have been accepted:
Check out the "How to make a report" section.