I have two pre-sentient species inside my territory, but they do not show up in the species screen, rendering me unable to uplift them. I have previously successfully uplifted a species (which went on to become an absolute majority in my empire, if you count all the genetic variations as a single species). I did disallow Native Enlightenment at one point to appease a bunch of xenophobic spiritualists, but allowing it again still doesn't get pre-sentients to appear on the list, even after waiting 10 years.
I don't remember if pre-sentients are supposed to appear as pops on the planet's Surface screen, but the two planets with the Stone Age Primitives modifier in my empire have no pops on them, perhaps they've been wiped out somehow, but the game failed to remove the planet modifier?
Update: I just found a pre-sentient population on a planet without a Stone Age Primitives modifier, whom I was able to uplift; that makes me think these are two separate systems, however I can not build an observation outpost on planets with that modifier, which you can do with sentient pre-FTL species. What can you do with those primitives?
Best Answer
Stone Age primitives are actually a third state in-between the pre-sentient races that you can uplift and the fully sentient primitive civilizations that you can observe.
This is explained on the Stellaris Wiki's Pre-FTL Species Page:
So you might not be able to interact at all. Perhaps they will advance to the Bronze Age at some point and you can observe them. It's not clear if that's a possibility.
If you can colonize, though, here's what you can do once you get there:
I think I actually had this happen on a planet in my current game, but I was confused because there was also a Pre-Sentient race on the planet. I uplifted that race, and was surprised to find that the Stone Age primitives continued to live in the reservations.