"She is a woman, not a machine."
Translation:Ona je žena, ne stroj.
This is a situation where I cannot see a reason why not yet at the same time I cannot imagine anybody actually saying it. Maybe because it is a longer combined sentence it just seems to be asking for ONA or at least TO "Je To zena, ne stroj". Just plain "Je zena, ne stroj" seems very awkward.
There is a grammatical difference between the sentences both in Czech and in English, although as a practical matter the ultimate meanings are not significantly different. One (this exercise) is "She is a woman, not a machine," while the second (your suggestion) is "She is a woman, she is not a machine." "Ona je žena, není stroj" is not accepted in the reverse exercise.