W:The repository 'http://ppa.launchpad.net/tualatrix/ppa/ubuntu xenial Release' does not have a Release file., W:Data from such a repository can't be authenticated and is therefore potentially dangerous to use., W:See apt-secure(8) manpage for repository creation and user configuration details., E:Failed to fetch http://ppa.launchpad.net/tualatrix/ppa/ubuntu/dists/xenial/main/binary-amd64/Packages  404  Not Found, E:Some index files failed to download. They have been ignored, or old ones used instead.