Sempre que tento executar uma atualização de software, recebo este erro do gerente de software:
W:GPG error: http://download.virtualbox.org/virtualbox/debian xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A2F683C52980AECF, W:The repository 'http://download.virtualbox.org/virtualbox/debian xenial InRelease' is not signed., 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., W:There is no public key available for the following key IDs:
A2F683C52980AECF , W:The repository 'http://ppa.launchpad.net/blackmage/f.lux/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., W:The repository 'http://ppa.launchpad.net/gezakovacs/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., W:The repository 'http://ppa.launchpad.net/zeal-developers/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., W:http://deb.playonlinux.com/dists/trusty/InRelease: Signature by key 74F7358425EEB6176094C884E0F72778C4676186 uses weak digest algorithm (SHA1), W:http://liveusb.info/multisystem/depot/dists/all/Release.gpg: Signature by key 32027DE3D67157C45E69C0AE4E940D7FDD7FB8CC uses weak digest algorithm (SHA1), W:http://vagrant-deb.linestarve.com/dists/any/InRelease: Signature by key AD319E0F7CFFA38B4D9F6E55CE3F3DE92099F7A4 uses weak digest algorithm (SHA1), E:Failed to fetch http://ppa.launchpad.net/blackmage/f.lux/ubuntu/dists/xenial/main/source/Sources 404 Not Found, E:Failed to fetch http://ppa.launchpad.net/blackmage/f.lux/ubuntu/dists/xenial/main/binary-i386/Packages 404 Not Found, E:Failed to fetch http://ppa.launchpad.net/gezakovacs/ppa/ubuntu/dists/xenial/main/binary-i386/Packages 404 Not Found, E:Failed to fetch http://ppa.launchpad.net/zeal-developers/ppa/ubuntu/dists/xenial/main/source/Sources 404 Not Found, E:Failed to fetch http://ppa.launchpad.net/zeal-developers/ppa/ubuntu/dists/xenial/main/binary-i386/Packages 404 Not Found, E:Some index files failed to download. They have been ignored, or old ones used instead.
Aqui estão algumas capturas de tela do que é exibido quando tento editar os repositórios que tenho:
Não tenho certeza de como consertar isso.