The skill challenges are always shown on the map even if you haven't explored the area. You are obviously missing a section somewhere so look around the edge of the map for ones that look close enough to be in the zone.
It took me forever to find the last one. Hopefully your missing the same one.
I havent got the game up (on my laptop) so sorry if i get some of the names wrong. North of that 'morgans spiral' there's a <3 task, you have to speak to the guy who will teleport you into a special area called biarthorn den i think. Once youre in there head to the back and left there's a passage which leads you around the back, to a skill point
Hope this is the one.. If not, I have 100% completion there so might still be able to help
Nobody cares about your PC specs, so please don't post them here.