"Where is your mother."
Translation:Где твоя мама?
Why is мать considered an error, when the word is 'mother'? As far as I know, мать means 'mother', while мама equals 'mom'.
Generally, it is as you said. However, when you address a child, it is uncommon to refer to their mother as "мать", "мама" sounds better in this context. Both should be accepted, though, I'd report it.
Мать is, unlike мама, a formal word, so with a pronoun ты you'd use мама, while for вы the varians мать sounds better, because then the formality level is balanced. :)