Check whether you have saved the message center number.

Listen, message not sent error happens only at the time when your message app can't find the message centre number, by default when you first insert the Sim inside the phone that number automatically saved in the app. Things getting worse when you lost it. Take out the Sim and put it into another phone. From there get the message centre number or lastly contact the service provider. I had the same issue at once and contacting the provider solved the problem.

you welcome. ????