"You are girls."
Translation:Vocês são meninas.
That's the first time I've seen anyone write a sentence using "vós". It seems correct to me. The problem is that in Brazilian Portuguese the 2nd person plural form is more or less confined to the Bible and possibly very flowery speeches or legalistic documents and is not used in everyday speech or writing (as far as I know).
You could ask for your answer to be accepted, but it is probably better to use the 3rd person plural form "vocês" in future so that you do not lose too many hearts.
By now Duolingo does accept Vós sois meninas, which I tried. However, I first tried Sois meninas, but this variant was not accepted. In Mozambique, I have noticed that the old 2nd person forms are used only for and among children and possibly within the closest family. In this example we have boys stating that they are boys, thus it would seem natural that they might use the traditional 2nd person form. Practice might be different in Brazilian Portuguese, which I know less about.
The English word "you" can be used to address one person - translated as the Portuguese word "você" (or "tu") - or a group of people - translated as "vocês" (or "vós"). In other words adding an "s" to "você" changes the meaning from singular "you" to plural "you".
So, as you are talking about a group of girls ("meninas"), you need to use "vocês" and the third person plural "são": "You are a girl" = "Você é uma menina", but "You are girls" = "Vocês são meninas".
Note: Although "tu" is used in some parts of Brazil, "vós" is very rare.
There are two verbs meaning "to be" in Portuguese: one is "ser" and the present tense version that goes with 3rd person plural subjects is "são", for example, "Eles são" (they are); the other is "estar" and the present tense version that goes with singular subjects is "está", for example, "Ele está" (he is).
For the different uses of "ser" and "estar" see: