Let me give you a couple example sentences -

Das ist ein Mann. (the subject is masculine, man)

Das ist eine Frau. (the subject is feminine, woman)

Notice what's happening? The "Das" stays the same, because in those sentences "Das" is analogous to "That" (i.e. That is a man / That is a woman) in English. And "that" is a neutral term as it can point to anything.

It is the ein/eine that changes according to the gender of the subject of those sentences (i.e. Man/Woman) in German.

To clarify further, if you're trying to say "He is a man / She is a woman" then you would say the following -

Er ist ein Mann.

Sie ist eine Frau.

Das is a demonstrative pronoun in this context, not an article—so it doesn't change to correspond with gender. I do, however, think that Duolingo German could do a lot better job of teaching gender by adding a row of skills devoted to the masculine, feminine, and neuter (and incorporating examples like your sentence in order to foster clarity). My OP is here, so if this resonates with you at all I would totally appreciate your thoughts: https://www.duolingo.com/comment/25701538