"It is a cat."
Translation:Es ist eine Katze.
Personal pronouns such as "it" refer back to something previously mentioned.
Without context, "it" would be the neuter es in German -- possibly refering back to das Ding or das Tier in a previous sentence.
After you've introduced the cat, you can refer back to it with the feminine sie, matching the feminine grammatical gender of the noun Katze.
Was ist das? Es ist eine Katze. Sie heißt Minka.
But Duo's sentence "Das ist eine Katze" is being offered as "It is a cat" or "That is a cat". Why doesn't it go both ways?
It shouldn't. There may be some inconsistencies depending on how old the sentence is and who added or last modified it, but in general, the course sticks to es = it, das = that.