There's already a bug report for this that you can vote on:
https://forums.ea.com/idea/the-sims-4-bug-reports-en/gtw-carrot-cake-cannot-have-candles-or-wedding-topper/12552007
I have noticed that if i have the cake not centered correctly on the counter, they bday candles options is not there until you center it right, try moving it around the counter to see if the candle option appears. You can also move it over to the a dining table, that seems to always work. I agree, cakes and other baked items should but listed under Bake!
I have noticed that if i have the cake not centered correctly on the counter, they bday candles options is not there until you center it right, try moving it around the counter to see if the candle option appears.
Unfortunately you still can't put candles on a carrot cake no matter where you place it. If you had placed another cake in the wrong spot, the option would still be there just greyed out.
Poor hamburger cake placed off-centre has greyed out candles optionNo candle options for carrot cake even when centred
