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.

Open Software & Updates and enable "Source code".

Here is a (currently untested) Bash script that could be used to uncomment deb-src lines in sources.list :

tempdir=$(mktemp -d)
cd "$tempdir"
source_file=/etc/apt/sources.list
new_file=sources.list.new
perl -pE 's/^#\s+(deb-src)/$1/' "$source_file" > "$new_file"
sudo cp "$new_file" "$source_file"
sudo apt-get update