ati@atideb:~$ sudo apt update
Hit:1 http://linux.teamviewer.com/deb stable InRelease
Hit:2 http://download.virtualbox.org/virtualbox/debian stretch InRelease
Ign:3 http://dl.google.com/linux/chrome-remote-desktop/deb stable InRelease
Hit:5 http://ftp.debian.org/debian stretch-backports InRelease
Ign:6 http://ppa.launchpad.net/pidgin-developers/ppa/ubuntu disco InRelease
Ign:4 http://cdn-fastly.deb.debian.org/debian stretch InRelease
Hit:7 http://cdn-fastly.deb.debian.org/debian-security stretch/updates InRelease
Hit:10 https://deb.opera.com/opera-stable stable InRelease
Hit:11 https://dl.winehq.org/wine-builds/debian stretch InRelease
Hit:8 http://cdn-fastly.deb.debian.org/debian stretch-updates InRelease
Ign:13 http://dl.google.com/linux/chrome/deb stable InRelease
Hit:12 http://cdn-fastly.deb.debian.org/debian stretch Release
Hit:15 http://dl.google.com/linux/chrome-remote-desktop/deb stable Release
Err:16 http://ppa.launchpad.net/pidgin-developers/ppa/ubuntu disco Release
  404  Not Found
Get:9 http://ppa.launchpad.net/remmina-ppa-team/remmina-next/ubuntu disco InRelease [15.9 kB]
Hit:17 http://dl.google.com/linux/chrome/deb stable Release
Hit:18 https://download.sublimetext.com apt/stable/ InRelease
Get:14 http://ppa.launchpad.net/team-xbmc/ppa/ubuntu disco InRelease [15.4 kB]
Err:9 http://ppa.launchpad.net/remmina-ppa-team/remmina-next/ubuntu disco InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8A993C2521C5F0BA
Err:14 http://ppa.launchpad.net/team-xbmc/ppa/ubuntu disco InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6D975C4791E7EE5E
Reading package lists... Done
E: The repository 'http://ppa.launchpad.net/pidgin-developers/ppa/ubuntu disco Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://ppa.launchpad.net/remmina-ppa-team/remmina-next/ubuntu disco InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8A993C2521C5F0BA
E: The repository 'http://ppa.launchpad.net/remmina-ppa-team/remmina-next/ubuntu disco InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://ppa.launchpad.net/team-xbmc/ppa/ubuntu disco InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6D975C4791E7EE5E
E: The repository 'http://ppa.launchpad.net/team-xbmc/ppa/ubuntu disco InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
ati@atideb:~$ sudo apt upgrade
E: dpkg was interrupted, you must manually run 'sudo dpkg --configure -a' to correct the problem.