As you have noticed during the Ubuntu installation there was no question about the root password, as you might have been used to see during other Linux distribution installation process. This is why the root account is inactive and can’t be used (no password configured) until we will setup a proper password for it. To do this, we simply need to run:
sudo passwd root
This will ask for a new root password and once you confirm it, you can start using the root account to login.
In case you will want to disable back the root account, just lock the root account by running:
sudo passwd -l root