An administrator at Northern Trail Outfitters is unable to add a new user in salesforce.

One of the possible reasons why an administrator is unable to add a new user in Salesforce is that the username is already in use by another user in any Salesforce org. Usernames must be globally unique across all Salesforce orgs, so the administrator needs to choose a different username for the new user. References: https://help.salesforce.com/s/articleView?id=sf.users_add.htm &type=5