Molto probabilmente il problema è che quell'e-mail è usata come e-mail di recupero per un altro ID Apple.
In base alla discussione del supporto Apple:
The issue was that I wanted to create an account with an address that
was already listed as a secondary address for an another, older, Apple
ID account.
This situation blocks the Apple ID creation process (and because the
email is not a primary address for any Apple ID yet, there is no
birthdate associated with it and the recovery emails go nowhere)
To solve the problem I went to id.apple.com, clicked "manage your ID"
and logged in with my old account credentials.
I then deleted the email address that I wanted to use to create an ID
from the list of "alternate emails" associated with the old account
and victory! After that the Apple ID creation process went fine.