Parece que isso acontece porque você usou CDs / DVDs para instalar o sistema em vez de repositórios online e, quando você tenta usar o gerenciador de pacotes, ele simplesmente se refere a esses CDs.
Você pode querer adicionar repositórios do Ubuntu em /etc/apt/sources.list ou /etc/apt/sources.list.d
O conteúdo padrão do conteúdo do /etc/apt/sources.list para o Ubuntu Precise instalado usando imagens de netboot está listado abaixo:
deb http://archive.ubuntu.com/ubuntu precise main restricted
deb-src http://archive.ubuntu.com/ubuntu precise main restricted
deb http://archive.ubuntu.com/ubuntu precise-updates main restricted
deb-src http://archive.ubuntu.com/ubuntu precise-updates main restricted
deb http://archive.ubuntu.com/ubuntu precise universe
deb-src http://archive.ubuntu.com/ubuntu precise universe
deb http://archive.ubuntu.com/ubuntu precise-updates universe
deb-src http://archive.ubuntu.com/ubuntu precise-updates universe
deb http://archive.ubuntu.com/ubuntu precise multiverse
deb-src http://archive.ubuntu.com/ubuntu precise multiverse
deb http://archive.ubuntu.com/ubuntu precise-updates multiverse
deb-src http://archive.ubuntu.com/ubuntu precise-updates multiverse
deb http://archive.ubuntu.com/ubuntu precise-backports main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu precise-backports main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu precise-security main restricted
deb-src http://security.ubuntu.com/ubuntu precise-security main restricted
deb http://security.ubuntu.com/ubuntu precise-security universe
deb-src http://security.ubuntu.com/ubuntu precise-security universe
deb http://security.ubuntu.com/ubuntu precise-security multiverse
deb-src http://security.ubuntu.com/ubuntu precise-security multiverse
Em seguida, execute apt-get update e instale o pacote buil-essential como tentou anteriormente.