Unable to add a new email alias on iCloud

Hi,

I have been trying to add a new email alias to my iCloud.com account by going to: https://appleid.apple.com/account/home

Every time I enter an email ID, I get the following error: "You can not use this email address. Please try again using a different email address." without giving me an explanation on why.

At first I thought the email alias was taken. However, I have tried all variations and still received the same error.

Appreciate your help in getting this resolved.

Thanks

Samih