TL;DR Update your apt sources.list for ROS 2 using the Setup Sources section of the Debian package installation guide.
Previous releases of ROS 2 have required users to add the ROS 2 Buildfarm repository host (repo.ros2.org) as an apt source repository. With the release of Crystal ROS 2 packages can now be found alongside the ROS packages on packages.ros.org in dedicated ros2 and ros2-testing repositories. All ROS 2 distributions, including Bouncy, Ardent, and even the betas, r2b2 and r2b3 are available at the new location. While the old urls aren’t going away, the new repository should offer better availability so everyone is encouraged to update at their earliest convenience.