You have to prefix the command sudo ...
or active root account that ubuntu has by default disabled, this page are the steps to do
For safety the root account is disabled by default in Ubuntu. Instead every time you want to execute a command with superuser permissions have to precede it with the sudo command, which allows you to run commands as root if we were another user (by default, but you can use the-u flag to specify another user).
However we may want to momentarily activate the root account if you ever have to perform many administrative tasks, so we do not have to type sudo dozen times.
To enable the root account just assign a password:
sudo passwd root
Once this is done we can log in as root, or log in as a normal user and start executing commands as root by typing the su command.
Upon completion of the tasks I needed to perform, it is desirable to re-disable the super user with the flag-l (-lock):