Plesk update fails: GPG error: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY

Symptoms

• Debian or Ubuntu is installed on the server.

• Plesk updates or some extensions installation fail with the following message:

  PLESK_ERROR: An error occurred on attempt to install packages.
  Attention! Your software might be inoperable.
  Please contact product technical support.

• Following warning message could be found in /var/log/plesk/install/autoinstaller3.log:

  W: GPG error: http://repo.cloudlinux.com/kernelcare-debian/8 stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY DC3D600CDEF74BB

  OR

  W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://us-west-2.ec2.archive.ubuntu.com/ubuntu xenial-updates InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32

  OR

  Executing: /tmp/apt-key-gpghome.dhoA48mqwL/gpg.1.sh --keyserver hkp://ha.pool.sks-keyservers.net:80 --recv-keys 58118E89F3A912897C070ADBF76221572C52609D
   , stderr: Warning: apt-key output should not be parsed (stdout is not a terminal) 
   gpg: key F76221572C52609D: 7 signatures not checked due to missing keys

Cause

Required public keys are missing.

Resolution

1. Login to the server via SSH.

2. Add missing keys by executing the following command for each key from the log file, for example:

   # sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-keys F76221572C52609D
   # sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-keys 40976EAF437D05B5
   # sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-keys 6DC3D600CDEF74BB

   Note: the actual key server name and the key mentioned in the error message should be used in the command above

3. In case of Docker installation failure, use the command below:

   # sudo apt-key adv –keyserver hkp://ha.pool.sks-keyservers.net:80 –recv-keys F76221572C52609D

4. Clean “apt” cache:

   # apt-get clean

5. Re-install Plesk updates using this guide.