The conditions are set up so that one of two things must be true. Either you need to have Late Feudal Administration (requires Conclave) or you need High Crown Authority (requires not having Conclave).
If you don't have Conclave and you do have High Crown Authority then either it's a bug or you're not fulfilling one of the other requirements (ruled for 10 years, at peace, no vassals are fighting each other, no count or higher vassal has a negative opinion of you, not under a regency, title is not a viceroyalty).
Another thing is that if you have Conclave and you have both Elecitve Monarchy and Full Council Authority you can only switch to Elective Gavelkind. I realize OP doesn't have Conclave, just listing this for the sake of completeness.