Hello, Cidneybug13!
Thanks for posting. I'm sorry to hear you're having difficulty logging into your AT&T Messages app. When are you seeing this error? Is it just when you try to log in from a computer? Or do you see it on all your devices?
If you're attempting to log in from a computer, have you tried using a different browser or deleting your cookies?
Let me know if you have any other details you can share so we can try to help.
-Mariana