SSH is a very powerful technology, though not very user friendly. Sometimes things need to be done at the “root” level. However, for security purposes a regular user cannot simply log in as root using “su -” if not granted access.
How do you add a user to this access? Simple: in WHM when you log in, go to Security Center, then Manage Wheel Group Users. Over there you can add users to this “Wheel Group” to get in as root.
Hope this helps!