ソフトウェアの更新を実行しようとするたびに、ソフトウェアマネージャーからこのエラーが発生します。
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.
以下に、所有しているリポジトリを編集しようとしたときに表示されるスクリーンショットを示します。
これを修正する方法がわかりません。