@Aurel - A screenshot would help, but my first guess is that your target pawn has a collision body or prop on it whose collision profile is set to WorldStatic or WorldDynamic and is therefore being picked up as a navigation obstacle by the system. That would explain why your origin/goal is always invalid.
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@jvukovich - Looks like you're reusing the sample behavior tree of the demo project. The demo project has been setup in such a way that it picks goals from pawns via the "Goal Actor" public variable (which is set on a pawn from the map via world outliner). Additionally, each pawn implements a simple interface called "Demo Pawn Interface" which allows the behavior tree task to extract the goal from any arbitrary pawn. Now, if you're testing with the sample BT and not doing either of these, then the "Invalid Goal" message which you're seeing appears.
Of course you will eventually want to switch out the sample “Pick Goal” / “Pick Bot” tasks with your BT task/logic/implementation which sets the FlightLocationKey blackboard to a suitable vector depending on the specific gameplay tasks your A.I. is trying to achieve.
Edit:- I notice you’re already setting the BB keys directly. In that case you should not be using the Pick Bot (“Refresh Pursuit Goal”) node in your behavior tree at all. That node will attempt to override the goal with its own logic which is not desirable in your case.