title: GPG Error Updating Ubuntu/Debian Packages
body: <p>I am attempting to update my Ubuntu/Debian system and I am getting an error:</p>
  
  <pre><code>GPG error: http://deb.opera.com stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY E585066A30C18A2B
  </code></pre>
  
  <p>What does it mean, and how can I resolve it?</p>