This question has already been answered in other posts. Below is my proposed solution:
This error is usually caused when there are not enough stories for rasa to learn what action to perform next. I also ran into this problem, and apart from adding more stories, what really solved it was to explicitly state in the rules that the bot should execute the action_listen after the action in which it was receiving the fallback. In my house I only had the fallback with a single action, so I only added it once. If it happens to you with many actions, this may not be the most elegant option, but it is functional.
IMPORTANT! Remember that if in the rules you indicate that the action_listen must be executed after an action, you must also add it in all the stories that after this action an intent is executed, so that the action_listen can be executed, this way the rule is not contradicted.
Another possible solution would be to add stories in which after the action that launches the Fallback, a user intent appears, so that Rasa would learn that after that action it must wait for a response from the user.
