From what I've seen, both the turret and the target must exist in the layout before adding a target will work.
In my case, the turret is part of many actions that can be taken during the game, and the turret only exists as part of the project, not the layout.
So when I add a target at the start it doesn't work, since neither the turret or the target exist.
If I put the add in the "on created" for my enemy (the target), it only works if the turret exists before the enemy is created.
So, I tried a family that includes my turret and my enemy, and a condition for the family "on created" to add my target if either the turret or the target is created. If I had the enemy on screen first, then created a turret, this didn't work. Only worked if I had the turret first, then created the enemy.
So then I put them in an OR block instead, with an "on created" for the turret and enemy, and the same action of adding the target as when I had the family condition set up. This time it worked, no matter what order they were in.
Not sure why it didn't work with the family condition set up, maybe I had it wrong. No big, the OR block works fine for my setup.