This is most likely to happen on a normal GUI system like Ubuntu or Linux Mint. If you or the user is meant to have sudo / root privileges it is as simple as editing the following files:
Now assume your username is "iamtheuser"
Find the above lines and add a comma and "iamtheuser" right after as shown in the example above. You'll have to log out and login and things will be fine after that. If you want to stay logged into the GUI you could always just ssh in to localhost and that ssh connection would give you the new privileges.