Internet Issue- Sometimes the internet is not working properly and this is the main reason for facing issues.
Wrong Emails- If you have entered your email address wrong then it’s confirmed that you are not able to login in to your account
Old and Outdated Browser- Sometimes your browser is not updated then the browser is not able to work properly.
read also: https://ityug.wordpress.com/2021/08/18/complete-guide-to-fix-att-email-login-issues/
https://www.mrjourno.com/technology/how-do-you-fix-at-t-email-login-issues-1628585122.html