The final collectible - a flower for Hugo's Herbarium - is located in Chapter 16 - King Hugo of Plague Tale: Requiem. Our walkthrough explains where to find it, therefore letting you fully complete Chapter 16.
Chapter 16 - list of collectibles
There is only one collectible to find in Chapter 16 of Plague Tale: Requiem. The collectible in question is:
- Hugo's Herbarium x1
Hugo's Herbarium #1 - Carnation
This secret can be easily found at the beginning of the chapter, as you're making your way through the ruined city. While following the main path you'll need to use the stairs pictured above.
Go a bit further. Lucas will briefly stop next to the flower. Interact with him.