Hello. My name is***** can assist you with your question.
There isn't any information here on what email accounts you are using or email software that you are using, however, the error message is usually related to using more than one account from a single email client and trying to create an alias for an account that already exists.
If you have two Outlook.com accounts, you can’t add this as an alias for the other since they are already Microsoft account.
If your third party account was already registered as Microsoft account, you cannot also add this as alias. You can only add an existing email address as an alias if it is not yet registered as Microsoft account.
If you want to manage several accounts in one mailbox, you can use mail client like Windows Live Mail. You can also use Outlook.com to send and receive emails from other account that uses POP. You may check this link for more details and how to do this: Send and receive email from other accounts in Outlook.com
So, if you are simply trying to use more than one account with an email client you can add both accounts to the email client. But if you are trying to add an alias, and that alias is not allowed, this is likely when you'll see the message. If your circumstances are different, please provide me with whatever information you can that will help me assist you.