If you can’t add a phone number that means that you already have an Autopay profile (perhaps with another email) with that phone number. You got three choices;
1. Complete a new profile without a phone number or sign in to existing profile. (Fyi: phone number is not a mandatory field, and the communication from us goes to your email).
2. Use forgot your password to get a link to get into your existing profile.
Ps: you can delete a profile if you have two by logging in, go to profile and scroll to the bottom to you see delete profile.