By default, if the new installer is utilized when deploying Ubuntu server 18.04, the /etc/apt/sources.list will contain only two entries. This doesn’t cover much, so to expand it to be a bit more useful we replace it with:
deb http://jp.archive.ubuntu.com/ubuntu/ bionic main restricted deb http://jp.archive.ubuntu.com/ubuntu/ bionic-updates main restricted deb http://jp.archive.ubuntu.com/ubuntu/ bionic universe deb http://jp.archive.ubuntu.com/ubuntu/ bionic-updates universe deb http://jp.archive.ubuntu.com/ubuntu/ bionic multiverse deb http://jp.archive.ubuntu.com/ubuntu/ bionic-updates multiverse deb http://jp.archive.ubuntu.com/ubuntu/ bionic-backports main restricted universe multiverse deb http://security.ubuntu.com/ubuntu bionic-security main restricted deb http://security.ubuntu.com/ubuntu bionic-security universe deb http://security.ubuntu.com/ubuntu bionic-security multiverse
Note that this is for Japan (“jp”). Change this to “us” or similar to match your region if not in Japan.