Thank you, that is a valid point. I have added the sentence: "Εγώ είμαι γυναίκα." but have maintained "Εγώ είμαι μία γυναίκα." There are two reasons for this. The first is that without context the use of the definite article is not incorrect. The other is that as many learners have seen this sentence it might cause confusion to find it only without the article.
In addition, I have made similar alterations to: " "Εγώ είμαι ένας άντρας" and "Εγώ είμαι ένα κορίτσι."
Thank you for your input and we look forward to more. We are working on a new tree in which we hope to have corrected the errors found on this tree. Please see the forum site to a make suggests here
Shouldn't "μια" be the right solution instead of "μία"? Or is the explanation in https://forum.duolingo.com/comment/17806268/%CE%BC%CE%AF%CE%B1-and-%CE%BC%CE%B9%CE%B1%3E not correct?