"No, I don't want potatoes."
Translation:Nac ydw, dw i ddim eisiau tatws.
I think that may be in the north-west Wales dialect, but I am not sure about other areas. The general answer to Wyt ti eisiau...? is Ydw/Nac ydw.
The Oes/Nac oes answer would be commonly used in response to the more formal question pattern 'Oes eisiau tatws arnat ti?'
If you live in a particular area of Wales then use, in speech at least, whatever local forms you feel most comfortable with. Unfortunately Duo cannot really deal with dialect variations to any great degree as there are so many of them in some contexts.
Duo will sometimes drop in phrases from earlier sections as a way of letting you check that you still remember them. This is also done by similar learning applications.
The basic negative pattern is explained in the 'Hints and Tips' notes for 'Present Tense 1'. What part of the pattern do you need help with?