Segue abaixo modelo do arquivo /etc/apt/source.list para o ubuntu 20.04 ou derivados:
deb http://archive.ubuntu.com/ubuntu/ focal main restricted universe multiverse deb-src http://archive.ubuntu.com/ubuntu/ focal main restricted universe multiverse deb http://archive.ubuntu.com/ubuntu/ focal-updates main restricted universe multiverse deb-src http://archive.ubuntu.com/ubuntu/ focal-updates main restricted universe multiverse deb http://archive.ubuntu.com/ubuntu/ focal-security main restricted universe multiverse deb-src http://archive.ubuntu.com/ubuntu/ focal-security main restricted universe multiverse deb http://archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse deb-src http://archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse deb http://archive.canonical.com/ubuntu focal partner deb-src http://archive.canonical.com/ubuntu focal partne
Elder Rodrigues
Leitor voraz e um dos administradores do GNU/Linux Brasil no Whatsapp, facebook, youtube e nesse dito site: www.gnulinuxbrasil.com.br