That quest is actually in beauclair, which you can only access through blood and wine not white orchard. You'll want to make your way to the gran'place area, where there's this vineyard owner hanging around behind trentin's workshop who's looking to settle up some payment with you. Interesting thing about this one, you don't need to have done any specific contracts beforehand even though he'll mention some past work you've supposedly done for him.

Good to know: just keep in mind you'll need the blood and wine expansion to even get there, and you'll have to progress through the dlc's main story a bit before you can start poking around beauclair properly. A lot of players get mixed up because there are notice boards all over the place, but this particular quest isn't one you'll find posted anywhere - you need to actually chat with the vineyard owner to kick things off.

Pretty straightforward really, though worth mentioning that anyone telling you to check near the armorer's shop is steering you wrong - stick to looking around trentin's workshop and the gran'place area, and you'll find your guy.