@karngenie Thanks for your response and sharing more details on this.
- As mentioned in the error message
az webapp deployment setcmdlet should be unique across the all the regions .
- Can you try running the cmdlet
az webapp deployment user showcmdlet and check whether this particular user is exists in your subscription or not. If this particular user is present in your subscription then you will be able to reset the password of that account.
- If you are having multiple subscription I would suggest you to run this
az webapp deployment user showcmdlet in all those accounts and check under which subscription/Azure account does this user is present.
- If this particular user is not present in any of your subscription since username should be unique this user name might be used by some other customer.