It's a result of the integration of "Enhanced Cultural Decisions". Basically integrate the culture that corresponds to the tradition, then a cultural decision to "Study the (Culture Group) Ways of war" should be there, and in order to keep the traditions avaliable, there needs to be a base amount of that culture's pops.

It looks like you're forced to keep them integrated too. Unlike the base game where once unlocked they stay unlocked here loosing too many pops or revoking their citizenship locks the tree again.

i had the right number of etruscians according to the tooltip, but it still didnt let me make the cultural decision or unlock the decision. Tried a larger pop group with double the numbers, sitll didnt work. Any way to specifically disable the 'enhanced cultural decisions' mod out of this modpack?