Change notification email address in Apple ID
How to change a notification email address in Apple ID account?
I'm trying to change my Apple ID with another email address (gmail.com). An error popped up saying that I cannot use an email address same as my "Notification Email Address". So, I changed the "Contactable Email Address". However, I am still not allowed to change the Apple ID with the new email address (i.e. gmail.com). I went back to check the "Contactable Email Address" and confirmed that the secondary address (gmail.com) is no longer existed. Why I still cannot use it as my new Apple ID?
If you know what's wrong, please let me know the proper procedure to solve this problem. Thanks.
MacBook Air (13-inch, Early 2015), iOS 10.2