The person you are trying to message has opted out of receiving messages from your Twilio phone number, Channels sender, or Messaging Service.
You have attempted to message a 'To' number that has replied with "STOP" to one of your previous messages. You will not be able to send to the phone number specified in the 'To' parameter until the subscriber identified by the phone number has responded with "START".
Please see this FAQ for more information about how Twilio handles opt-out and opt-in.
The end user handset has responded with "STOP" or another opt-out keyword.