The To attribute does not appear to be a phone number that you can SMS.
- Invalid formatted number
- SMSing an un-validated number while using a Trial Account
- Attempting to send a message where the "To" number is the same as the "From" number
- Check the number and make sure it is a valid 10 digit domestic phone number
- Confirm you have validated this number, if you are still using a Trial account, or upgrade your account
- Contact Twilio customer support to inquire further.