Hello MSmallsr,
Thank you for reaching out to Apple Support Communities. We are happy to help you!
We understand that you are unable to send an SMS message when using one of your iPad devices, but the other iPad seems to work just fine.
Have you made sure that your software is up to date on the specific iPad that you are having issues with?
Update your iPhone, iPad, or iPod touch
If everything seems to be up to date, and the issue persists, all the necessary troubleshooting steps for the issue at hand would be found in the article that you connect to us through, If you can't send or receive messages on your iPhone, iPad, or iPod touch. Have you attempted all steps suggested in this article already?
If so and the issue continues, we would suggest to Contact Apple Support for additional assistance.
We hope this finds you well!
Cheers!