W: The repository 'http://ppa.launchpad.net/skunk/pepper-flash/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/skunk/pepper-flash/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.