Explanation
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

Explanation
Minimum password length and password complexity requirement are two settings that administrators can configure to increase the security of user passwords in Salesforce. They determine how long and how complex the passwords must be to meet the security standards. Password different than username and prevent common words are not valid settings in Salesforce, although they are good practices for creating strong passwords.
References: https://help.salesforce.com/s/articleView?id=sf.admin_password_policies.htm&type=5