"Repository does not have a release file" error

I'm trying to install [ROS][1] on Ubuntu. I'm getting this error: sudo apt-get update Hit:1 http://pk.archive.ubuntu.com/ubuntu cosmic InRelease Hit:2 http://pk.archive.ubuntu.com/ubuntu cosmic-updates InRelease Hit:3 http://pk.archive.ubuntu.com/ubuntu cosmic-backports InRelease Get:4 http://security.ubuntu.com/ubuntu cosmic-security InRelease [83.2 kB] Ign:5 http://packages.ros.org/ros/ubuntu cosmic InRelease Err:6 http://packages.ros.org/ros/ubuntu cosmic Release 404 Not Found [IP: 64.50.236.52 80] Reading package lists... Done E: The repository 'http://packages.ros.org/ros/ubuntu cosmic Release' does not have a Release file. N: Updating from such a repository can't be done securely, and is therefore disabled by default. N: See apt-secure(8) manpage for repository creation and user configuration details. [1]: https://en.wikipedia.org/wiki/Robot_Operating_System