Can I ask if you have tested a different messaging app to see if the error occurs there as well?
Basically many ATT owners report this issue.
There are 2 ways that the issue could be eliminated and one of the below steps or possibly both should be able to make it go away.
1) Go to settings>tap on magnifying glass> and type reset. From there select reset network setting. Just know that this will reset your known wifi networks and make you re-pair bluetooth devices.
2) From another post: "I went to message settings and stopped backing up to the cloud and the popup has stopped"