The specified index doesn't exist, therefore cannot be queried.
Possible Causes
You may have made a typo.
You may have made a programming error that yields the empty string in place of a valid index name.
Possible Solutions
Please review https://www.twilio.com/docs/sync/live-query for the currently supported list of indices, and make sure your code consumes one of those. There is no facility to create your own index today.