Exam Salesforce-Certified-Administrator Topic 2 Question 240 Discussion

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