So, I had the same problem, and finally got it to work. Apparently, when you first download Norton from Xfinity, it creates a Norton account using your xfinity email address, email@example.com as the user name. That is the account you need to sign into when asked--you can't use another Norton account. You may have to reset the password--not sure if it's the same as your Xfinity password. Hope it works for you.
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