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.
Lumina Spine of Keres Treasure
What you want to do is transmat in the Divalian Mists region of Dreaming City, and go towards the Spine of Keres. Then, follow the left path away from the palace and follow it until you reach a neat walkway/bridge behind the palace itself. Move across and look to the left, where you'll see a stone island of sorts - jump there, and search it thoroughly.
The chest you need to open to complete this step of the Lumina quest will be hidden behind a small bush to the left side of this rocky outcropping, and if you get lost, just use the map given above.
If the flip of the coin says you should look for this very same treasure on Titan's Sinking Docks instead, fret not - we've got that location covered, too. Good luck!