KSkillMax

[Noctis]

There is one single reason for it, you don't have any party member in slot 3 (alias index 2) as the script call pretends so it will obviously end up throwing that error message at your embarrassed face. Poor you! I should have warned you about that just in case...

I tested it again. I used script call (below) while I had only aluxes in my party
$game_party.actors[1].skill_limit +=1
That script call is for aluxes as far as I know but I still get the same error.
Btw. is it normal that skills vanish when I put them out?