Unable to add a second email to my Apple Account

Thank you for your reply. Sorry nothing above has helped.

You might want to try contacting the Apple Account Support Team: (eg: online chat / phone / email etc.}

There is a "Get support - start now" link at the bottom of their webpage (link below)

Apple Account - Official Apple Support

Alternatively: You can choose your country : Apple phone numbers around the world

See: Contact Apple Support - Apple Support

Wondering where you are unable to add that email address:

You may need a computer or device with a recent operating system (iOS / MacOS / iPadOS)

or

You can not use an email address that is already associated with a different Apple Account.

Else:

You can sign in to your Apple Account online. You should be able to manage all your account details there:

Log-in webpage: https://account.apple.com

If needed:

See how to: Manage and use your Apple Account - Apple Support

and

More info: About your Apple Account email addresses - Apple Support

Eg: says:

Add an additional email address

1. Sign in at account.apple.com.
2. Select Sign-In and Security.
3. Choose Email & Phone Numbers, then select the Add button.
4. Enter your additional email address. This additional email address can’t already be associated with a different Apple Account.
5. Apple will send a verification to that address. Didn’t get the email?
6. Enter the verification code and select Continue.

I had this issue with a custom domain. I suspect it had something to do with when I first tried to register that particular iCloud account, something glitched. Afterwards, it was just showing the same error you got. I was able to register any number of other iCloud accounts to that domain, but never that one.

While I never needed to resolve it, I think the best answer would be to contact Apple Support and see if they can disassociate the domain from that iCloud account.