"Os gweli di'n dda ga i afal?"
Translation:Please may I have an apple?
It is just the form of words for 'please' to use with ti, the informal singular form of 'you', rather than chi, the formal or plural 'you'.
"May I have an apple?" is marked incorrect. Is there a subtlety to the translation that I am missing?
Did you include the 'please'? It is required here. Otherwise, 'may I' and 'can I ' should probably both be acceptable in this context - remember that the course is still in beta, so things are still to be ironed out.