nano /etc/ssh/sshd_config
2) Add a line in the Authentication section of the file that says PermitRootLogin yes. This line may already exist and be commented out with a "#". In this case, remove the "#".
# Authentication:#LoginGraceTime 2mPermitRootLogin yes#StrictModes yes#MaxAuthTries 6#MaxSessions 10
3) Save the updated /etc/ssh/sshd_config file.
4) Restart the SSH server:
service sshd restart
source: 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
Common Issues:
If the root have locked by too many wrong authentications. Try:
usermod -U root