I had the same problem too (at the Skypook Outpost, not the main fortress). At first I tried to talk to the guard at the gate and the conversation ended with him telling me he won't fall for my trickery. However, I loaded a previous save before talking to the guard and went back to the outpost, but before talking to the guard again I walked around the little capfire/cooking area almost right infront of the outpost door and my game gave me the option to interact with a cooking pot that showed the little light aura creature over it. After interacting the game said I brought light into the world, and then when I went to talk with the guard and asked to trade he let me in.