In order to allow access of system's su (superuser) utility to users, It is required to add them on Wheel group user.
1) Login to your WHM using the root user.
2) Go to Security -> Manage Wheel Group Users.

3) Select the user from list and click on add to group.
Accuwebhosting.com
1) Login to your WHM using the root user.
2) Go to Security -> Manage Wheel Group Users.
3) Select the user from list and click on add to group.
Accuwebhosting.com