άνδρας is in the incubator and of course should have been accepted. The trouble is that some sections of Duo e.g. Strengthen skill, multiple choice, listening, test out etc do not receive all our edits and reject correct words. Please let us know which section you encountered this in. It will help a lot.
I did some "refreshers" in "accusative case" and "determiners". That's where I may have used άνδρας vice "άντρας".
Συμφωνα με το λεξικο του Μπαμπινιωτη ειναι εξ' ισου σωστα και το ΑΝΤΡΑΣ και το ΑΝΔΡΑΣ στη Νεοελληνικη Γλωσσα. Το ανδρας μου το εβγαλε λαθος στη απαντηση ενω ΔΕΝ ειναι .
As you can see from the comments on this page we are aware that both are correct and we accept either. The problem is the Duobot that does not always follow our directions. It's a technical error on the part of Duolingo. Thank you for your input.