我正在尝试在Ubuntu 上安装ROS。我收到此错误：
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 …