How to fix the error "user" is not in the sudoers file. This incident will be reported."

In order not to have this message anymore, you need to had "user" to the sodoers file.
To solve this issue,  you need to log in a terminal as root (command: su) and then type the command visudo.
In the file it appears, locate the line containing:
 
root    ALL=(ALL)       ALL

and right below, add the line:

"user"    ALL=(ALL)       ALL
Where, obviously, "user" is the name of the user you want to add.
Close the file (ESC :w and :q  OR just give ESC :x)

No more incidents to be reported! :)