Issue with changing Apple ID
Most likely the problem is is that that email is used as a recovery email for another Apple ID.

According to this Apple Support discussion:


  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.