can't upgrade it at the Gypsum Kiln. The Mechanic shows up for example
Complete the following to improve your F i n i s h e r
Defeat Vanash at Tribunal Highmound
To unlock the Empty Socket.
Defeat -30- 10 Sporebound Boar enemies in Elysian Wilds
To unlock Deadly Flourish.
Defeat 30 Crowned Rotseed at Isle of Zurvān
To unlock Keen Tondo.
Upgrade this item at the Gypsum Kiln ¹
To see which perks can be applied, check out the "Perk Bucket" Tab in the Finisher article.
@evzn make sure it isn't equipped or in a gear set.