The ‘Peasant’ trait is automatically given when you load into a low income lot! There is no need to add it in CAS or through an interaction.

It can also be cheated through the Royalty Mod pie menu.

(In order for the Peasant trait to be added to any sim (including NPCs) you must enter the lot once. If you are setting up a royal themed save, enter into every lot you want the trait to apply to - this includes middle class and gentry.)