Sorry you're having this trouble. I do not believe a third-party app would be the solution. I'd like to ask a few questions to better troubleshoot this for you:
Are you sending to an International number?
Is the error message happening with all messages you try to send, or just one specific number?
If one number, how is it programmed into your phone (i.e. 1+area code + number or just area code + number)?
Also if one number, have you been able to send to this number in the past?