System updates cannot be installed in Plesk due to Imunify repositories: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY

Symptoms

Plesk admin received the following email:

CONFIG_TEXT: Reason: 2023-04-06 06:26:11 INFO: pum is called with arguments: ['--list', '--repo-info', '--json']
2023-04-06 06:26: 13 ERROR:
2023-04-06 06:26:13 ERROR: Exited with returncode 1.

In Plesk > Tools & Settings > System Updates page the following error is present:

PLESK_ERROR: 2023-04-07 10:35:54 ERROR: W:GPG error: https://repo.imunify360.cloudlinux.com/imunify360/debian/11 bullseye InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8C55A6628608CB71, E:The repository 'https://repo.imunify360.cloudlinux.com/imunify360/debian/11 bullseye InRelease' is not signed., W:Updating from such a repository can't be done securely, and is therefore disabled by default., W:See apt-secure(8) manpage for repository creation and user configuration details.

System updates via apt command cannot be installed with the same error.

Cause

Misconfiguration of Imunify repositories.

Resolution

1. Connect to the server via SSH

2. Update Imunify Agent license:

   # imunify360-agent update-license

3. Add the Imunify repositories to the system:

   # wget -O RPM-GPG-KEY-CloudLinux https://repo.imunify360.cloudlinux.com/defense360/RPM-GPG-KEY-CloudLinux
   apt-key add RPM-GPG-KEY-CloudLinux<