is your character a part of the religion/culture required to join that society

Yes, though I believe that part of the problem may be that I was using characters I created, rather than the base ones. I was able to join the Agni Kai as the base characters, so I belive that was the issue. A solution that would allow custom characters would be greatly appreciated however.

I've checked the conditions, and for some reason, it seems to think I don't have the firebender trait, even if I have taken it.