"Está enfermo. Por eso no ha venido."
Translation:He is sick. That is why he has not come.
Assuming you were phoning to let someone know the excuse you've given for them not coming to the place you are at. I believe they should accept that answer. If you explain the circumstance you believe your sentence can be used to DL they will update the app to accept the answer. If it can be translated as you then there's a reason for it.
this is what it said the answer was: "He's sick. that's why she hasn't come." That makes no sense! WHY does it say He in the first sentence, and She in the second one??? I put "She is sick. That is why she hasn't come." And it was marked wrong. How does one know if esta is to be for a male or female???????
If you check the comments you will see that it's not está that indicates the gender of the subject, but enfermo. It must be either "you" (usted) or "he" as the subject of the first sentence. The second is ambiguous and could be "she." Duo appears to have corrected only the part you got wrong and left "she" in the second part of your translation.
There are contexts where the gender might shift between the two sentences. Perhaps, we're describing the actions of a couple and the female partner is staying home with her male partner because he's sick. That would not be unusual in my opinion.