Explanation
To identify why a user is unable to log in to Salesforce, the administrator should review the login history for the user. The login history shows the date and time of each login attempt, the source IP address, the browser and platform used, the login type (such as username and password or single sign-on), and the status (such as success or failure). The login history can help troubleshoot common login issues such as incorrect username or password, invalid security token, IP restrictions, or login hours violations. Checking the attempted logins by running the setup audit trail, pulling the password history, or resetting the security token for the profile will not help identify why a user is unable to log in. References:
https://help.salesforce.com/s/articleView?id=sf.monitoring_login_history.htm&type=5

Explanation
Assignment rules are tools that allow administrators to automatically route records to users or queues based on certain criteria. For example, an assignment rule can assign cases to different queues based on case priority, origin, type, or other fields. Assignment rules can be triggered when records are created manually, via email, web, or API. Assignment rules consist of multiple rule entries that define the criteria and actions for each assignment scenario. References:
https://help.salesforce.com/s/articleView?id=sf.customize_leadrules.htm&type=5https://help.salesforce.com/s/ar