Why I cannot login remote server with its root

# ssh root@192.168.122.96
root@192.168.122.96's password: 
Permission denied, please try again.

Do according to [1].

NOTE: on Ubuntu, remember to restart ssh service like this “sudo restart ssh”.

[1] https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/v2v_guide/preparation_before_the_p2v_migration-enable_root_login_over_ssh