I am currently playing the PC version and have 3 unused perk points which I can't seem to allocate.
I have the required skillpoints and prerequisites, and I can navigate the tree menus, but I can't activate any of the perks.
I've tried every key on the keyboard but none of them work. I would have thought it would be the "use item" key, but no.
Any suggestions? Thanks.

Just had a thought...
Not in front of the game right now so I can't test it.
Do the "novice" perks at the bottom of each tree have to be activated to advance further?
I thought they were already active because they are white, not grey.
But maybe I have to spend a perk point on those first.
Can anyone confirm?
Thanks.

yes they have to be activated before you can advance further and you need a certain number of points before you can level them up. Also some perks you can keep level up 5 times

Cheers.