You may have to enable multiverse [EDIT: in fact universe is required, not multiverse] :
deb http://ports.ubuntu.com/ubuntu-ports/ xenial main restricted universe multiverse
deb-src http://ports.ubuntu.com/ubuntu-ports/ xenial main restricted universe multiverse
deb http://ports.ubuntu.com/ubuntu-ports/ xenial-updates main restricted universe multiverse
deb-src http://ports.ubuntu.com/ubuntu-ports/ xenial-updates main restricted universe multiverse
deb http://ports.ubuntu.com/ubuntu-ports/ xenial-security main restricted universe multiverse
deb-src http://ports.ubuntu.com/ubuntu-ports/ xenial-security main restricted universe multiverse