Answer by Tobi for How to automatically update /etc/apt/sources.list with source URIs on Ubuntu 16.04?

I had this same issue on a server install of Ubuntu 16.04, so no GUI. All that I needed was a couple of sed commands.

sudo sed -i -- 's/#deb-src/deb-src/g' /etc/apt/sources.list && sudo sed -i -- 's/# deb-src/deb-src/g' /etc/apt/sources.list

Then sudo apt-get update and continue on.