I saw the latest update to TruePVE:

Added Players Can Trigger Traps With No Owner (true)

I have this set to true. I placed a flame turret and then used the Abandoned Bases plugin to make the base abandoned. I checked and the base and the flame turret have no owner. The flame turret is not attacking.

What do I have to do to get it (and other traps like regular turrets, land mines, etc) to work?

hi, those are turrets. there is no option for that, only traps which are guntraps, landmines and beartraps.

also, flame turrets from abandoned bases do not require this. they target by default, and require nothing special. something else is blocking those.

you can try this one for turrets, https://pastebin.com/shZvWQya