How do I enable the source code repositories?

You can enable source code repositories by uncommenting (removing #'s) deb-src repositories from /etc/apt/sources.list.

sed -i '/deb-src/s/^# //' /etc/apt/sources.list && apt update

If you want to disable source code repositories, you can comment it back

sed -i '/deb-src/s/^/# /' /etc/apt/sources.list && apt update

Launch the software center and select software sources:

In the software sources make sure "Source Code" is checked:

The default archive repositories have the source archives enabled as well as the binaries. Also, adding any PPAs with sudo add-apt-repository ppa:user/archive should enable the source and binary archives both.

To verify, you can check in the /etc/apt/sources.list file, and in the files in /etc/apt/sources.list.d/, to ensure the deb-src lines for each deb archive line, are there. Also, in the Software Sources properties dialog, you can check that the Source code option is checked on the Ubuntu Software tab, and that there are corresponding Source entries for the Binaries entries, in the list of additional sources under the Other Software tab.