surprise! did you ever make your AI move?
are you using navigation mesh?
did you set your character BP to use that AI?
did you run your behavior tree at begin play?
if you watched all that tutorials you must have solved your problem!
as you said you need your AI to just aim and walk around when the player is not in his sight. in that tutorial I sent you first he implemented an AI that walks around and looks for player if player is not in his sight. so you can use the same system and replace AI walk animation with an aiming walk animation.