This error will occur only in the case of Enterprise Bots where a bot is expected to converse with the users within the particular organization(selected users). Even within the organization also, the bot will be able to communicate with the users those who have been given with appropriate privileges.
Unless the Bots Enterprise Admin assigns the bot or respective task on specific user, the user will not be able to communicate with it.
In order to be able to talk to the bot, please work with your Bots Enterprise Admin and have the bot or task assigned. Thank you.