title: Ubuntu 16 R The following signatures couldn't be verified
body: <p>While updating the package, I am getting below error.</p>
  
  <pre><code>sudo apt update
  
  
  Reading package lists... Done 
  W: GPG error: https://cloud.r-project.org/bin/linux/ubuntu xenial/ InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 51716619E084DAB9
  E: The repository 'https://cloud.r-project.org/bin/linux/ubuntu xenial/ 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.
  </code></pre>
  
  <p>I have tried below solutions but none of it works.</p>
  
  <p><strong>sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys E084DAB9</strong></p>
  
  <p><strong>gpg --keyserver keyserver.ubuntu.com --recv-key E084DAB9</strong></p>
  
  <p><strong>gpg -a --export E084DAB9 | sudo apt-key add -</strong>  </p>