We can definitely assist you with getting started on the myAT&T app. We understand that you were prompted to link your accounts when you attempted to Create your AT&T User ID. Do you already have a User ID associated with the phone number that you entered while trying to create a new User ID? If so, you will have to change the phone number associated with that User ID before you can proceed with creating the new one, or you can use the existing User ID instead. If you're not sure about already having a User ID or not, or you forgot it, you can Learn how to recover your AT&T Access ID here. Let us know if this helps.
Sean, AT&T Community Specialist
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*I am an AT&T employee, and the postings on this site are my own and don't necessarily represent AT&T's position, strategies or opinions.