Hmmm…
Well, I was able to produce successful results upon replicating your setup. Perhaps it’s about some settings you’ve tinkered with, or there’s an error in the implementation.
I’d recommend creating a new project (in my testing I used the third person template) and carefully implementing the necessary steps. You might also wanna advance in stages, by first placing the nav mesh and executing an AI Move To node on Begin Play, then doing the same inside your custom AI controller, then creating a blackboard and a behavior tree, now running the behavior tree on Begin Play inside the custom AI controller, and triggering a custom task in the behavior tree that prints an on screen message, after that you can move on with creating a new object type blackboard key in your blackboard, setting its base class as the actor class, setting its value as the player character, and using it in a Move To task, if everything works fine you can then use a different actor as the target, and finally you can add your custom decorator as the condition (but I’d recommend setting the target on interaction and checking if it’s valid instead of having an obsolete bool type variable for it)
If you encounter any issues in the progress, make sure to share all the steps you’ve taken to get there, and at which point the issue arises.
Let us know how it goes 