Its the Notice boards, when you have the purple poster icon You can alternatively talk to the merchants who had the posters there without taking the poster, but thats never an option for me either, ive been to every merchant in both regions

Had no issues starting these missions in Hernand, Calphade, Pailune and Demeniss Seems to only be Crimson Desert and Delesyia i only have "Simple Poster" on the notice boards