You're in the wrong scope for the province, yeah. If you wanted to mess with a province you'd need to go back into province scope with "location". However, if it's a province event that's what your ROOT will be so...Try something like this in your option tags:
Code:option = { name = "EVTOPTA10000001" add_province_modifier = library }
I think that'll work.
EDIT: I'm not familiar with using negatives for MTTH. That might be invalid? How is this event supposed to be triggered?
That's what I tried first, simply having an option that adds a province modifier crashes the game. The trigger should target a province and check for libraries in the baronies, correct? Then the option adds a modifier to the province which was checked for libraries. Somehow the trigger isn't targeting the province like it should.
Having a negative (I've only ever tried -1 days or months) causes the event to fire as soon as the triggers are met.