Definite articles in Greek are equivalent to the English word ''the'', however, in Greek, they vary depending on the gender and number of the word that follows.
Note it is the word that has the gender which may not be the same as what is described. So, as above there is "Ο άντρας"(man) masculine gender but "το αγόρη" (boy) neuter gender, "η καρέκλα" (chair) feminine gender.
Please see the TipsNotes on the exercise page for more information.
On the web course, each skill is accompanied by "Tips & notes" about the grammar, vocabulary etc of that skill. They are on the homepage right below the list of lessons.
If you can get to the web sometimes you can access them. HEre are some other resources you might find helpful. https://www.duolingo.com/comment/22040507
And of course, we are here for any questions you have.
From the sticky The Greek Alphabet::
How to add the accent. On the Greek keyboard first type the key on the right of the Λ (L) (the semicolon :/;) then type the letter you want the accent on. On a mobile just hold the letter for an extra second or two and there will appear a variety of that letter with accents. Slide up to the one you want.
Both άντρες and άνδρες are correct and accepted in this sentence (and all sentences except dictation.")
So, if this was an exercise where you listened to the sentence and it used "άντρες" you would have been required to us "αντρες". However, if this was not a listening/dictation exercise there might have been another error.
If you could give us more information or even better a screenshot we could be of more help.