The Spare Body Selection Box has been added to the game as per the 2/1/2023 patch in NIKKE. It allows you to select a spare body of all units, except for Pilgrims, limited units, and units after Laplace (for now). However, there are many reports where Commanders are accidentally selecting the WRONG spare bodies (or unwanted spares) when choosing a unit from the body selector. This is not a bug, but more of a bad UI design. The Selector window is extremely sensitive to touch/swipe. If the user accidentally scrolls on the (+) icon, it registers immediately. The user is also unable to scroll on the white spaces in-between the spare body icons (at least on mobile) [fixed]. Please double check if the unit you WANT is selected. Video below shows sensitivity of touches via scrolling, as well as, the inability to scroll on the white spaces in-between the spare bodies [fixed].

This only works as a dupe right? I cant choose a new unit?
Yes, if you select a unit that you don’t have, you will NOT obtain her as a unit. just her dupe materials to limit break
How do I obtain it? I already have lv141+ nikkes but idk what to do or where to find it
It’s located in the challenges tab