Hi, I have defeated every boss on each difficulty (at least I think) but the challenge floor isn't showing up at all or on the notice board at all. What am I missing?

You need to meet Hellhound in the dungeon of Old Ashton, so that she appears as a boss on the advanced floor once. Once you've defeated Hellhound in the advanced dungeon of the Spire, the note on the Aldlyn city board should appear.