Hello!
This is Tin, I will do my best to help you. Could you provide me with the exact error message you are receiving? It appears that you are trying to add a new alias to use this as your primary account. Please note that you can only add an alias or email address that is not already a Microsoft account.
Please refer to the link below for more information:
https://support.microsoft.com/en-us/outlook/add-or-remove-an-email-alias-in-outlook-com
Let me know if you have any further questions. I am here to help.
Thanks,
Tin