Here is a photo of three addresses on the "Email & Phone Numbers" page of the "Sign-in & Security" page. Notice the second address has a question mark. There is no option to remove that address. Which appears as the primary address on iCloud Mail settings.
You cannot delete or change a primary iCloud address. You can only do that with an alias.
Add and manage email aliases for iCloud Mail on iCloud.com - Apple Support - "Note: You can’t delete or turn off a primary iCloud Mail address. You can’t use an alias to create a separate Apple ID, and an email alias can’t be converted to a primary iCloud Mail address."
You cannot delete or change a primary iCloud address. You can only do that with an alias.
Add and manage email aliases for iCloud Mail on iCloud.com - Apple Support - "Note: You can’t delete or turn off a primary iCloud Mail address. You can’t use an alias to create a separate Apple ID, and an email alias can’t be converted to a primary iCloud Mail address."
Unable to remove email address
