I had a similar problem when I recently tried Watch ESPN on my home PC. I have Suddenlink, not AT&T, so I contacted them and they talked me through it.
First thing you have to do once you log into Watch ESPN is look near the top right of the page.
Mine says Suddenlink with a down arrow to the right. Yours should say AT&T U-verse (or whatever it is) with a down arrow next to it.
Hover your cursor over the down arrow, or click on it, and you will see SIGN OUT. Click on the SIGN OUT. That's right - you have to sign out of your provider first.
Seems weird that you must sign our of your provider first in order to Watch ESPN, but that's the process.
When the next page comes up, enter what you want to watch in the search box, e.g. Tennessee vs. Gonzaga, if you want to search for something specific.
Or, you can just click on the arrow in the center of the video screen if you want to watch what's there. Right now, it's something about the NBA.
After you do that, a list of providers will pop up: Click on your provider [AT&T U-Verse].
The next screen will say, Log-in to Verify: AT&T U-verse should be in a box to the right side. Click on Continue in that box.
Then the Watch ESPN video screen will pop up again. If you click on the arrow in the middle of the screen, the video will play.
But there will probably be a commercial first.
That's what works for me, anyway. Hope it works for you.