apt-get build-dep is unable to find a source package

As others have already noted, make sure that for every deb … entry in /etc/apt/sources.list and /etc/apt/sources.list.d/*, you have a matching deb-src … entry. The rest of the line must be identical.

The deb entry is for binary packages (i.e. ready to install), the deb-src is for source packages (i.e. ready to compile). The reason why the two kinds of packages are separated is that they are managed very differently: binary packages have a dependency tracking mechanism and a currently-installed list, whereas source packages are only tracked so that they can be downloaded conveniently. Note that when discussing package repositories, the word source means two unrelated things: a source as in a location to download packages from, and a source package as opposed to a binary package.

libgtk2.0-0 is the name of a binary package. It is built from a source package called gtk+2.0. The reason source and binary package names don't always match is that building a source package can produce multiple binary packages; for example, gtk+2.0 is the source for 14 packages as it is split into two libraries (libgtk2.0, libgail), corresponding packages to build programs using these libraries (…-dev), documentation for developers (…-doc), companion programs (libgtk2.0-bin), etc.

You can see the name of the source package corresponding to a binary package by checking the Source: … line in the output of dpkg -s BINARY_PACKAGE_NAME (if the package is installed) or apt-cache show BINARY_PACKAGE_NAME.

You can list the binary packages produced by a source package with aptitude search '?source-package(^SOURCE_PACKAGE_NAME$).

The command apt-get source downloads a source package. If you give it an argument which isn't a known source package, it looks it up in the database of installable binary packages and tries to download the corresponding source package.

The command apt-get build-dep follows the same approach to deduce the name of a source package, then queries the source package database to obtain a list of binary packages (the list in the Build-Dep: field), and installs those binary packages.

The Software Sources GUI has a checkbox “enable repositories with source code” for official repositories, make sure that it's ticked. If you add third-party repositories manually, make sure that you add both deb-src and deb lines.

CLI method

sudo sed -Ei 's/^# deb-src /deb-src /' /etc/apt/sources.list
sudo apt-get update

Tested on the Ubuntu 18.04 Docker image. After this change, I can successful do for example:

sudo apt-get build-dep hello

and before the change it failed with the error:

E: You must put some 'source' URIs in your sources.list

See also: https://askubuntu.com/questions/496549/error-you-must-put-some-source-uris-in-your-sources-list/857433#857433

software-properties-gtk option "Source code"

From the GUI, you can run:

software-properties-gtk

then under the "Ubuntu Software" tab click "Source code":

This has added some deb-src lines under /etc/apt/sources.list, and now I can do sudo apt-get build-dep <package>.

Tested on Ubuntu 16.04 to 18.04.

Thanks to @steeldriver, I figured out what to do.

Just had to add the following ling to my /etc/apt/sources.list:

deb-src ftp://ftp.de.debian.org/debian/ stable main contrib

deb-src repositories contain sources packages (as opposed to binary, ready-to-install packages) needed for compiling.

Edit: After Adding this, you must execute sudo apt-get update