It looks like you are logged in.
I think you mean you can't do "root" (administrator) commands using su.
su is not sudo.
If there is a sudoers file, and a user is in it. You can type sudo and then the command.
Many times sudoers limits users to specific commands. Some linux distro's such as Ubuntu
let you run all root commands. (Make you wonder why they even have sudo)
example:
sudo fdisk -l
However if you actually want to become another user, then you only type su. su stands for "switch user". You can do this two different ways.
1. su root -
In order to do this, you have to know the root password.
2. sudo su -
In order to do this, you have to have full root permissions in the sudoers file.
By default, redhat does not have any users in the sudoers file.
So unless someone added some accounts to the sudoers file during installation
(or sometime afterward) you will not be able to use sudo commands.
You can type sudo -l to see what commands you have access to. (Lowercase L)
If this says you don't have sudo access. The only other option is that you MUST know
the root password.