W: The repository 'http://ppa.launchpad.net/colingille/freshlight/ubuntu xenial Release' does not have a Release file. N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch http://ppa.launchpad.net/colingille/freshlight/ubuntu/dists/xenial/main/source/Sources  404  Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.

sudo add-apt-repository -r ppa:colingille/freshlight
sudo apt update