"The capes are mine."
Translation:As capas são minhas.
You were not wrong this time. It is Duolingo that apparently in over 5 years has failed to add the article to the database of correct answers.
See, in Brazil the articles with possessives are often optional (less so in all the other Portuguese speaking places which covers far more than just Portugal). So, despite that Duo will occasionally mark us down for it (their mistake, not ours), it is probably better to use the articles – because they are not wrong even in Brazilian Portuguese – than to leave them out.
As you get a better feel for the language, if the Brazilian dialect is the one you prefer then you will eventually get a better sense of when the articles are needed and when they can be dropped.