Not saying that this is the case in this example because sometimes Duo does run into computer problems. But there is something to consider.
The computer is programmed to look for mistakes. Some mistakes cannot be identified by it. It can spot a mistake but can't determine the origin.
Eg: You enter an incorrect but valid word which is a different gender from the correct answer. The computer sees the article you correctly attached to the word you chose and sees it is different from the gender of the correct answer. It stops there and reports the gender of the article is wrong. You look at it and say ...but the gender matches the noun....True, but it isn't the correct article gender if you had chosen the correct noun.
This applies to gender, number, agreement between modifiers and parts of speech. The computer may scan along a sentence and report the first of two related errors. Of course, for you the real error is you completely mistranslated the second one.
If you run into one of those cases where there is clearly nothing wrong with the supposed error look to see if you didn't make a mistake elsewhere that may be related in ways that you are not aware of.
This is especially true if, as in this case, hardly anyone else is reporting it.
Here is something to try. If you use the Chrome browser, go to their app store and download ..Duolingo Notes. It is an extension that sits up in your toolbar that lets you single click save Duo questions complete with your answer. That way you can recall it later for perusal by yourself or by posting it on the comments section here to see if there isn't something you missed. Also Duolingo Notes is very helpful in general. It is very useful when you get a q. and a. that seems to identical to a previous question but with different results. Then it is a simple matter to go back and compare the two letter by letter, if necessary, to see what differences there are. Having established the not so obvious difference, you can then copy and paste the two examples into your Anki deck and drill to kill.
Try listening to authentic German people pronouncing it. http://www.forvo.com/word/m%C3%A4dchen/#de
A little question: why "orange juice" is "Orangensaft" and "apple juice" is "Apfelsaft"? Is Apfel uniform? P.S. I'm asking this because I googled and got Äpfel as plural of apples. If Äpfel is right, could someone also explain me about the Umlauts use, mainly in nouns with prefix, sufix or compose with more than 1 word?
Orange receives an ending -n when in compounds, not because it is plural. The reason for that is because it is so, haha, there's no real logic behind this other than it is the way it is. So yes, you say Apfelsaft, but you say Orangensaft, even though both are singulars.
That's my understanding though, I stand to be corrected.
First of all, there are basic rules. Nouns that end in with a consonant mostly get e. Nouns that end in e get en. Mädchen is not plural and the en in Mädchen has nothing to do with this rule. "Chen" on the end shows endearment/smallness like das Schwein to das Schweinchen. If a noun ends in chen it is always neutral, das.
Plurals are not quite as regular in German as in English. However, there are some "guidelines" that are pretty useful.
For example words that contain only one syllable in the singular often gets an umlaut at the stem vowel in the plural.
die Axt (the axe) becomes die Äxte in plural, for example.
die Nuss (the nut) -> die Nüsse (the nuts).
der Mann (the man) -> die Männer (the men).
der Gott (the god) -> die Götter (the gods).
But there are of course many exceptions to this, for example, "der Tag" becomes "die Tage" (not Täge) in plural even though it only has one syllable.
And "der Hund" becomes "die Hunde" just like you point out (not Hünde like one would expect).
Words that contain two or more syllables are a bit rarer, but they do also exist.
Mutter - Mütter, Tochter - Töchter, Werkstatt - Werkstätten etc.. you have to remember by heart, but I think there are not as many of them as the first exception.
To better understand this. Example: The fish are in the water. Note the word "fish" and not "fishes". German also has it's own words that can be used as singular and plural. Example in plural form: The fish are hungry so I fed them. "Them" is what made the fish more than one. Mädchen is a equivalent to "Fish" or even "Deer". What makes it singular or plural is how it is presented (the or they) so Das Mädchen isst(the girl is eating) or Die Mädchen essen(the girls are eating).
It isn't always the word alone that suggests numbers but, as in English, can also be how it is presented by the rest of the sentence. ~_^