"User" is not in the sudoers file on CentOS

When you try to do a command that required root privilege like this

[phuongvu@localhost Desktop]$ sudo cat /etc/sudoers

We trust you have received the usual lecture from the local System
Administrator. It usually boils down to these three things:

    #1) Respect the privacy of others.
    #2) Think before you type.
    #3) With great power comes great responsibility.

[sudo] password for phuongvu: 
phuongvu is not in the sudoers file.  This incident will be reported.

A solution for this is add a current user in sudoers file. This file located in /etc/sudoers and you must use root privilege to edit this file.
Let do this:

Normally, sudoers file is read-only, therefore, we have to change access permission for it. Login as root user

[phuongvu@localhost Desktop]$ su
Password: 
[root@localhost Desktop]# chmod 666 /etc/sudoers

Edit sudoers file

[root@localhost Desktop]# vi /etc/sudoers

Find this line and add new line like below

## Allow root to run any commands anywhere
root    ALL=(ALL)       ALL
phuongvu        ALL=(ALL)       ALL

Save and re-grant read-only property for it

[root@localhost Desktop]# chmod 440 /etc/sudoers

Done. Now logout root account and use this command as normal user

[phuongvu@localhost Desktop]$ sudo cat /etc/sudoers