To enable AAA, you need to configure the aaa new-model command in global configuration.
Note: Until this command is enabled, all other AAA commands are hidden.
Warning: The aaa new-model command immediately applies local authentication to all lines and interfaces (except console line line con 0). If a telnet session is opened to the router after enabling this command (or if a connection times out and has to reconnect), then the user has to be authenticated using the the local database of the router. To avoid being locked out of the router, we recommend that you define a username and password on the access server before starting the AAA configuration.
https://www.cisco.com/c/en/us/support/docs/security-vpn/terminal-access-controller-access- control-system-tacacs-/10384-security.html