I'm sorry to hear that Showtime isn't letting you log in. Showtime/MaxGo requires the same email login that you use to view att.net for email, att.com for account info, uverseonline.com, etc. One thing that could help resolve this issue is logging into the att.net website first just to help re-authenticate what services you have and then try Showtime once more on your device to see if it will allow you access.
I hope this helps! Let me know how it goes and if it still doesn't work then we'll find another solution for you. Thank you!
- MorganCS - ATTU-verseCare
If you encounter any issues with your service or equipment, I recommend checking out our Troubleshoot & Resolve solutions to help diagnose the issue.