Hello @Yasmindanae,
Having issues while trying to login can be worrisome, especially if you are trying to check your bill or usage online. It will be my pleasure to help!
Clearing the cache and cookies on your browser can determine if it is a browser related issue. You can also use an alternate browser and see if you get the same error.
I am glad you are able to get in through your mobile device, so you can view the information you want. If the above steps still have not resolved the issue you are receiving, please send a private message to @ATTMobilityCare with your full name, mobile number, and what kind of message displays for you when trying to login.
Best regards!
Brett, AT&T Community Specialist