Candles cannot be put on any block unless the block you want to place it on is a solid block. Compare this to Java edition where you can even put a candle on lightning rods and end rods (see attached image) You can’t put them on upside down stair blocks or slabs either. In the attached videos, you can see that I can’t put a candle on a wall, upside down stair, lightning rod, and a slab since those items aren’t considered solid blocks. Oddly, there’s no issues with placing 4 on a smithing table since it is a solid block.