Hi,
You are getting this error because the tracker object of run function is a rasa_sdk tracker which does not contain trigger_followup_action.
The method trigger_followup_action is part of DialogueStateTracker which is a class of rasa.core.trackers.
I suggest you return an event for calling a followup action like below:
class ActionHelloWorld(Action):
def name(self) -> Text:
return "action_hello_world"
def run(self, dispatcher: CollectingDispatcher,
tracker: Tracker,
domain: Dict[Text, Any]) -> List[Dict[Text, Any]]:
dispatcher.utter_message("Hello World!")
return [rasa_sdk.events.FollowupAction("your_custom_action")]