Can't Change / Remove iCloud Notification Email Address

Hi all

I am trying to change the notification email address on my account. The email address will soon be erased. Each time I try to either verify or remove the email address on appleid.apple.com, I am presented with this response.

I don't see anywhere in my settings on my Mac or my iPhone where this address is shown, or can be changed / removed. How do I do this?

Thanks.