W:The repository 'http://extras.ubuntu.com/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://mirrors.nl.eu.kernel.org/ubuntu/dists/xenial/InRelease Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?), E:Failed to fetch http://mirrors.nl.eu.kernel.org/ubuntu/dists/xenial-updates/InRelease Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?), E:Failed to fetch http://mirrors.nl.eu.kernel.org/ubuntu/dists/xenial-backports/InRelease Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?), E:Failed to fetch http://mirrors.nl.eu.kernel.org/ubuntu/dists/xenial-security/InRelease Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?), E:Failed to fetch http://extras.ubuntu.com/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found [IP: 91.189.92.152 80], E:Some index files failed to download. They have been ignored, or old ones used instead.