From GameWatcher: “Destiny 2 is no stranger to wild goose chases, and the Lumina quest (and Spine of Keres) is one of the more erratic ones you could come across. We’ll tell you where you should look for the hidden treasure you’ll need for this questline.
The Spine of Keres hidden treasure can spawn on a wide variety of locations, which includes Titan, for example. That said, you’ll find the exact spot on which the relevant chest is located down below, just be sure to reference the map.”
Source: N4G PC Destiny 2 Spine of Keres Lumina Chest Location