Estou recebendo a seguinte sequência de erros ao executar sudo apt-get update :
Ign:1 http://dl.google.com/linux/chrome/deb stable InRelease
Hit:2 http://ppa.launchpad.net/elementary-os/stable/ubuntu xenial InRelease
Ign:3 http://extras.ubuntu.com/ubuntu precise InRelease
Hit:4 http://dl.google.com/linux/chrome/deb stable Release
Ign:6 http://archive.canonical.com/ubuntu precise InRelease
Hit:7 http://ppa.launchpad.net/n-muench/burg/ubuntu xenial InRelease
Ign:8 http://us.archive.ubuntu.com/ubuntu precise InRelease
Get:9 http://extras.ubuntu.com/ubuntu precise Release [11,9 kB]
Hit:10 http://archive.canonical.com/ubuntu precise Release
Hit:11 http://ppa.launchpad.net/noobslab/macbuntu/ubuntu xenial InRelease
Get:13 http://us.archive.ubuntu.com/ubuntu precise-security InRelease [55,7 kB]
Hit:14 http://ppa.launchpad.net/elementary-os/os-patches/ubuntu xenial InRelease
Get:15 http://extras.ubuntu.com/ubuntu precise Release.gpg [72 B]
Ign:15 http://extras.ubuntu.com/ubuntu precise Release.gpg
Hit:16 http://ppa.launchpad.net/philip.scott/elementary-tweaks/ubuntu xenial InRelease
Ign:18 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial InRelease
Hit:21 http://ppa.launchpad.net/videolan/stable-daily/ubuntu xenial InRelease
Get:22 http://us.archive.ubuntu.com/ubuntu precise-updates InRelease [55,7 kB]
Hit:17 http://extras.ubuntu.com/ubuntu precise/main Sources
Ign:17 http://extras.ubuntu.com/ubuntu precise/main Sources
Hit:19 http://extras.ubuntu.com/ubuntu precise/main amd64 Packages
Ign:19 http://extras.ubuntu.com/ubuntu precise/main amd64 Packages
Ign:23 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial Release
Hit:20 http://extras.ubuntu.com/ubuntu precise/main i386 Packages
Ign:20 http://extras.ubuntu.com/ubuntu precise/main i386 Packages
Hit:24 http://us.archive.ubuntu.com/ubuntu precise-backports InRelease
Ign:25 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main amd64 Packages
Hit:17 http://extras.ubuntu.com/ubuntu precise/main Sources
Ign:26 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main i386 Packages
Hit:27 http://us.archive.ubuntu.com/ubuntu precise Release
Hit:19 http://extras.ubuntu.com/ubuntu precise/main amd64 Packages
Ign:29 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main all Packages
Hit:20 http://extras.ubuntu.com/ubuntu precise/main i386 Packages
Ign:30 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main Translation-en_US
Ign:31 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main Translation-en
Ign:32 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main amd64 DEP-11 Metadata
Ign:33 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main DEP-11 64x64 Icons
Ign:25 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main amd64 Packages
Ign:26 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main i386 Packages
Ign:29 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main all Packages
Ign:30 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main Translation-en_US
Ign:31 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main Translation-en
Ign:32 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main amd64 DEP-11 Metadata
Ign:33 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main DEP-11 64x64 Icons
Ign:25 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main amd64 Packages
Ign:26 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main i386 Packages
Ign:29 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main all Packages
Ign:30 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main Translation-en_US
Ign:31 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main Translation-en
Ign:32 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main amd64 DEP-11 Metadata
Ign:33 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main DEP-11 64x64 Icons
Ign:25 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main amd64 Packages
Ign:26 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main i386 Packages
Ign:29 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main all Packages
Ign:30 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main Translation-en_US
Ign:31 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main Translation-en
Ign:32 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main amd64 DEP-11 Metadata
Ign:33 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main DEP-11 64x64 Icons
Ign:25 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main amd64 Packages
Ign:26 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main i386 Packages
Ign:29 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main all Packages
Ign:30 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main Translation-en_US
Ign:31 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main Translation-en
Ign:32 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main amd64 DEP-11 Metadata
Ign:33 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main DEP-11 64x64 Icons
Err:25 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main amd64 Packages
404 Not Found
Ign:26 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main i386 Packages
Ign:29 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main all Packages
Ign:30 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main Translation-en_US
Ign:31 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main Translation-en
Ign:32 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main amd64 DEP-11 Metadata
Ign:33 http://ppa.launchpad.net/versable/elementary-update/ubuntu xenial/main DEP-11 64x64 Icons
Fetched 123 kB in 35s (3.456 B/s)
Reading package lists... Done
W: http://archive.canonical.com/ubuntu/dists/precise/Release.gpg: Signature by key 630239CC130E1A7FD81A27B140976EAF437D05B5 uses weak digest algorithm (SHA1)
W: GPG error: http://extras.ubuntu.com/ubuntu precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 16126D3A3E5C1192
W: The repository 'http://extras.ubuntu.com/ubuntu precise Release' is not signed.
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.
W: http://us.archive.ubuntu.com/ubuntu/dists/precise-security/InRelease: Signature by key 630239CC130E1A7FD81A27B140976EAF437D05B5 uses weak digest algorithm (SHA1)
W: http://us.archive.ubuntu.com/ubuntu/dists/precise-updates/InRelease: Signature by key 630239CC130E1A7FD81A27B140976EAF437D05B5 uses weak digest algorithm (SHA1)
W: The repository 'http://ppa.launchpad.net/versable/elementary-update/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.
W: http://us.archive.ubuntu.com/ubuntu/dists/precise-backports/InRelease: Signature by key 630239CC130E1A7FD81A27B140976EAF437D05B5 uses weak digest algorithm (SHA1)
W: http://us.archive.ubuntu.com/ubuntu/dists/precise/Release.gpg: Signature by key 630239CC130E1A7FD81A27B140976EAF437D05B5 uses weak digest algorithm (SHA1)
E: Failed to fetch http://ppa.launchpad.net/versable/elementary-update/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.
O que esses erros significam e como posso corrigi-los?
Meu conteúdo do arquivo sources.list é
#############################################################
################### OFFICIAL UBUNTU REPOS ###################
#############################################################
###### Ubuntu Main Repos
deb http://us.archive.ubuntu.com/ubuntu/ main restricted universe multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ precise main restricted universe multiverse
###### Ubuntu Update Repos
deb http://us.archive.ubuntu.com/ubuntu/ precise-security main restricted universe multiverse
deb http://us.archive.ubuntu.com/ubuntu/ precise-updates main restricted universe multiverse
deb http://us.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ precise-security main restricted universe multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ precise-updates main restricted universe multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse
###### Ubuntu Partner Repo
deb http://archive.canonical.com/ubuntu precise partner
deb-src http://archive.canonical.com/ubuntu precise partner
###### Ubuntu Extras Repo
deb http://extras.ubuntu.com/ubuntu precise main
deb-src http://extras.ubuntu.com/ubuntu precise main
Da sua saída apt-get update , vejo que você tem precise repositórios no Xenial, que é a origem do problema. Você deve remover essas linhas do seu arquivo sources.list .
Primeiro abra o arquivo usando este comando em um terminal
sudo -i gedit /etc/apt/sources.list
Em seguida, remova todas as linhas que têm precise .
Em seguida, salve o arquivo e saia do gedit. Agora, faça sudo apt-get update .
O problema deve ter desaparecido.