Hello,
Is the username for this account from outlook.com or hotmail.com?
According to your description, it sounds like the email address/alias was deleted. Did you delete it?
If so, once it is deleted from a Microsoft account as an alias, that email address is gone permanently. Microsoft doesn't reuse its own email address, so it cannot be added to created as alias for another Microsoft account.
Microsoft explains it here: https://support.microsoft.com/office/add-or-remove-an-email-alias-in-outlook-com-459b1989-356d-40fa-a689-8f285b13f1f2
Under the section "Remove an email address", it says: "Are you sure? If you remove an alias that's an email address from a Microsoft domain (like @hotmail.com, @live.com, @outlook.com, or @msn.com), you're permanently deleting the alias and it can't be associated with any Microsoft account again."