Translation:The birthday cake will be ready Saturday at two o'clock.
Why can’t I say fourteen hundred hours? that matches more closely what was said in French.
It's very un-English to say fourteen hundred hours in the middle of a conversation unless it's some kind of military op I guess. I think Duo won't accept this because it wants the translation to be converted from good French (they use 24-hours when speaking about time) to good English (we usually use 12-hours when speaking).