Not able to use iCloud mail or create @icloud.com

I created an Apple account for a family member using phone number about a month ago. After the whole setup and software updates (now running on iOS 18.1.1), I opened iCloud mail to create an @icloud.com email. It showed iCloud mail is not available for your account. I then opened the browser and signed in to iCloud.com mail, and there also the same error was shown. I then watched some tutorials on Youtube, and went to settings and clicked on the name, went to iCloud to toggle on 'Use iCloud mail on this device', but there was no button like that. Also in list of apps using iCloud there was no Mail app. I have restarted the iPhone and reinstalled the Mail app but nothing helped. In the sign-in and security page there is also no option to create a new email - it only gives an option to add an existing email address. I'm now using a Gmail account as primary mail temporarily but I want to create an @icloud.com email address. Device - iPhone 15