I can't change my apple id
when I try and change my Apple ID, it says "you can only add the @icloud.com address that is already on your file account". what does this mean?
MacBook Air 13″, macOS 10.14
It indicates that the only option to change the email address is to use the iCloud email address that in already on your account. You can't use an iCloud email address that isn't on your account.
See if anything here helps.
What to do after you change your Apple ID or password - Apple Support I suggest doing this before you change your Apple ID or password.