administrator at Northern Trail Outfitters is unable to add a new user in Salesforce. What could cause this issue?

One of the possible causes for being unable to add a new user in Salesforce is that the username is

already in use by another organization. Usernames must be globally unique across all Salesforce orgs,

so if another user has claimed that username before, it cannot be used again. To fix this issue, choose

a different username that is not taken by anyone else. The username is not restricted to a domain

specific to my domain unless specified by an administrator. The email address used for the username

does not have to match a contact record. The email used for the username can be any valid email

address. Reference:

https://help.salesforce.com/s/articleView?id=sf.admin_usermgmt_add.htm&type=5