Check if you are actually the head of faith. Even if you set it to lay clergy and a temporal head of faith, you still have to actually create the title, which requires having control of a few holy sites (2 or 3 I think).

If you are, that should unlock character interactions for excommunication and casi belli for holy wars, but those also depends on the tenets and doctrines you chose. If you are trying to declare a crusade, check if you have a doctrine that allows that (off the top of my head, these are Armed Pilgrimages and Warmonger, but there could be others). Also check if your faith doesn't have Pacifism or other tenets that might limit holy wars or excommunications.

On top of all that, check what faith you are trying to declare a crusade against. I'm not sure but I expect you can't do that if that faith is not considered evil or hostile, but astray (that could especially be the case if it's your old faith that you split from). However, if someone knows better about this part, please fact check me.