I just had this same problem and an att agent could not resolve the issue. I had to figure it out on my own and it was very confusing. I dont understand why I cant just use my ATT account info to login to watch tv and hbo go.
When linking your hbo go with att you need to link it with your watchtv password and id. Dont worry about providing your email when prompted by the webpage, use the watchtv user id instead. That is what solved my issue. Very confusing.
You have to scroll past all the first set of choices and find the option to "show all providers" (or something like that). You will then be presented with a different list, and on that list you will Find AT&T WatchTV. It is very counter-intuitive.