How can use iCloud reserved email address?

Hi,

I have received an email from "iCloud" along below reservation message:

"An @icloud.com email address has been reserved for you"

as I can't disclosed actual given email address here but lets say "reserved@icloud.com" actually previously I had "reserved@me.com" so that's why this email address held for me and now I am trying to create new account with following steps but not able to do so:

www.icloud.com [login with me Apple ID] > Mail > Preference > Accounts > Add an Alias = Here I am trying to create new reserved mail address but below message is showing:

"This Alias is not available"

Thanks