Straight dump of the error message:

Err:8 http://packages.ros.org/ros/ubuntu disco/main amd64 Packages
  404  Not Found [IP: 140.211.166.134 80]
Ign:9 http://packages.ros.org/ros/ubuntu disco/main Translation-en_US
Ign:10 http://packages.ros.org/ros/ubuntu disco/main Translation-en
Ign:11 http://packages.ros.org/ros/ubuntu disco/main amd64 c-n-f Metadata
Ign:12 http://packages.ros.org/ros/ubuntu disco/main all c-n-f Metadata
Reading package lists... Done
E: Failed to fetch http://packages.ros.org/ros/ubuntu/dists/disco/main/binary-amd64/Packages  404  Not Found [IP: 140.211.166.134 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.