Hi 08-31-40,
Welcome to the Apple Support Communities and thanks for posting your issue here! We'd be glad to take a closer look into this with you. We understand that you're experiencing issues when you're attempting to send messages from your secondary line. We see that you've already checked to make sure you're following the correct steps in the Related Article section. Have you also checked to see if you're experiencing similar issues making or receiving phone calls from your secondary line? Were you able to send messages from your secondary line previously? Have you also contacted your carrier to ensure there isn't anything on their end that could be preventing you from using your secondary line?
Cheers.