How to enable the root login in the ESX server which is OFF by default?

Edit the configuration file for SSH with the command:

nano /etc/ssh/sshd_config

Find the line that starts with PermitRootLogin and change the no to yes. You can find this line about 2 pages down from the top.

Save the file by first pressing Ctrl-O and then Enter.

Exit with Ctrl-X.

Restart the sshd service with the command:

service sshd restart