When the NLP is not able to categorize the user's input into a specific intent, a standard block is used to inform the user that the input input has not been correctly identified.

This kind of block is called Fallback block.

By default the content of the fallback block is as follows: 

'Sorry, could not find an answer for you. Please try again."

The CMS gives you the option to create custom fallbacks, you can read how to do that HERE