When you install Ubuntu, it doesn’t give you the option of creating a root account. Only a user. Ubuntu is more desktop oriented so it pretends to not actually have a root account. This only works so long as you don’t legitimately need it for something. If you do, there is a simple trick to get a root shell.
Pop open a terminal window and type:
sudo passwd
You will be prompted for your current account’s password. Next you will be prompted to set the root password and confirm it. Now type:
su
You should be prompted for the password you just created and then you will be handed a root shell for your efforts.