By default, if your bot doesn't understand a message, it won't reply to it.
If you wish to add a custom reply for those messages, go to the Customer care section and click on Any Unrecognized Message:
Enable autoresponse here:
Simply add your response or a few responses the bot will choose between. These will be sent to your followers when the bot doesn't understand a message.
NOTE: If you just enable this option and don't add any custom responses your bot will display this ️emoji for all messages it doesn't understand.
In these response variations, you can also add images, audio, video, carousel, an external link or link to your features.