Use "move to’ node in the behavior tree and set the blackboard key to a ‘Target Actor’, and only allow execution if the blackboard value ‘Target Actor’ is set. You could set and clear the ‘Target Actor’ from within the AI controller if you are using AI Perception.
Also you might want to put the ‘Default Focus’ service on the ‘move to’ if you want your AI to focus on your ‘Target Actor’, this will allow you to rotate the AI to face the ‘Target Actor’, or you could simple check the ‘Observe Blackboard Value Tolerance’ within the ‘move to’ node to check every so often if the ‘Target Actor’ has changed location.
Finally you can run a check by using a built in decorated called ‘Is At Location’, so you can do something when your AI reached the ‘Target Actor’.