Cannot add alias to iCloud Mail

See the article Add and manage email aliases for iCloud Mail on iCloud.com - Apple Support

After you set up a primary iCloud Mail address, you can create up to three @icloud.com email aliases on iCloud.com. After you set up an alias, you can send and receive mail from that alias on iCloud.com and on any device that has Mail turned on in iCloud settings.

If you set up a custom domain with iCloud+, you can change the full name that appears with your custom domain addresses in Mail preferences. You can create and manage your email addresses for that domain on the iCloud+ Features page on iCloud.com.

Note: If you had five MobileMe email aliases when you moved your account to iCloud, you would retain the five aliases. However, if you delete one of your five aliases, you cannot create a new one to replace it.

Hello TheShaunSaw,

Thanks for your post to Apple Support Communities. We understand you are trying to save an iCloud alias, but getting an error message. The first thing we'd recommend trying is to log out of iCloud completely, restart your Mac. Then, log in to the iCloud website again and see if you can save the alias.

If that still doesn't help, next we'd recommend reaching out to our support teams directly. They can gather further details and work through additional steps with you. You can reach them here: Get Support

You can also find our phone number for your region at this link: Contact Apple for support and service

Best regards.