By default Ubuntu server root account is disable, if we want to execute a command that needs administrative permission we need to use the sudo command.
This is a good security practice however this might not work for you.
In order to enable the root account and not use the sudo command all we need to do is enable the root account by setting a password.
This is what we need to do, In the command line type:
sudo passwd root
After typing the command you will have to set the password.
In case you would like to disable the account again you will have to issue the following command:
sudo passwd -l root