The word "igjen" has several meanings. It can be used to say "again", "once again" or "once more", but it's also used to express how much remains of something. In the context of this sentence, the latter meaning is the more likely one.
If the situation were to repeat itself, you could say something like this:
"Det er bare ett kakestykke igjen igjen."
"There is only one piece of cake left again."
If we wanted to say 'there is only one piece of cake left over', would we add any preposition corresponding to 'over' to this sentence? I know it doesn't alter the meaning very much, but Norwegian and English are so closely related that sometimes very small differences like this correlate one to one. Tusen takk!
The emphasis in the pronunciation of this sentence lies on the word 'ett', pressuring the fact that there's just one piece. If the emphasis had lied on 'kakestykke', it would've been written 'et' - in that case the fact that there's a piece of cake left is more important than the number of slices.
In English it seems very different to me - just one piece of cake emphasizes the one, just a piece of cake emphasizes the cake (maybe there was a dessert platter with various things, but all that's left is cake).
If it was a written exercise (one where you're given a written sentence to translate), you'd be correct. Since it isn't (it's a spoken exercise where the sentence is spoken and you have to translate it), there is no pronunciation difference between "et" and "ett", and both words make semantic and grammatical sense in the sentence, in my opinion you're incorrect, and it does not matter at all.
There is a slight difference in pronunciation between 'et' and 'ett' (the latter is pronounced with more emphasis, higher pitch). I do agree that both should be accepted if it was a spoken exercise, but Duolingo does not allow for separate translations for spoken and written exercises, nor does it allow homophones (unless this has changed).