Ubuntu 18.04 Bionic Beaver installation comes with a blank root password as default. This is a security precaution as user is expected to never login as the root user. For any privileged administration tasks the user is required to use the sudo command.
This is a companion discussion topic for the original entry at https://linuxconfig.org/how-to-set-a-root-password-on-ubuntu-18-04-bionic-beaver-linux