Unable to upgrade Plesk on Debian or Ubuntu: Failed to fetch autoinstall.plesk.com/... 404 Not Found

Symptoms

• Plesk server is running on a Debian-based OS (Debian 9/10, Ubuntu 18.04/20.04)

• One of the following errors is displayed while upgrading the server:

  CONFIG_TEXT: E: Failed to fetch http://autoinstall.plesk.com/ubuntu/PSA_17.8.11/../../PSA_17.8.11/dist-deb-Ubuntu-16.04-x86_64/bootstrapper/sw-engine-cli-2.24_2.24.9-ubuntu.16.04.180409.1642_amd64.deb 404 Not Found
  E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

  CONFIG_TEXT: The repository 'http://my.archive.ubuntu.com/ubuntu focal Release' no longer has a Release file., W:Updating from such a repository can't be done securely, and is therefore disabled by default

  Note: the PSA and OS versions in the URLs may vary depending on the server's OS and current Plesk version

• The following error is displayed during component reinstallation:

  # plesk installer --select-release-current --upgrade-installed-components
  ...
  E: Version '17.8.11-ubuntu16.04.build1708180329.14' for 'pp17.8.11-bootstrapper' was not found
  E: Version '17.8.11-ubuntu16.04.build1708180322.11' for 'plesk-completion' was not found
  E: Version '17.8.11-ubuntu16.04.build1708180328.17' for 'plesk-repair-kit' was not found
  E: Version '17.8.11-ubuntu16.04.build1708180321.15' for 'psa-locale-base-en-us' was not found
  E: Version '17.8.11-ubuntu16.04.build1708180326.12' for 'psa-updates' was not found
  E: Version '17.8.11-ubuntu16.04.build1708180328.11' for 'plesk-l10n' was not found
  E: Version '17.8.11-ubuntu16.04.build1708180321.20' for 'psa-watchdog' was not found
  E: Version '17.8.11-ubuntu16.04.build1708181211.16' for 'psa-locale-base-en-us' was not found
  E: Version '4.8.4-ubuntu16.04.build1708181212.18' for 'psa-phpmyadmin' was not found

Cause

Repository misconfiguration in /etc/apt/sources.list.

Resolution

1. Connect to the server via SSH.

2. Back up the original /etc/apt/sources.list file:

   # mv /etc/apt/sources.list{,.orig}

3. Recreate the /etc/apt/sources.list file using vi text editor with the default repositories for server's OS: