Unable to Log In to HBO Max

I tried to “sign in to HBO Max with provider” using AT&T, and the login is not working (I’m getting Care Code 201 [LU100]), but when I tried the “Don’t have a user ID? Make one now” option and it told me “That account’s already registered” when I input my account info. If it’s already registered and I’ve never logged in before, shouldn’t the login be the same as my AT&T login? Please advise on how to fix this issue.