Download cacert.pem.txt

If you get this error while installing/updating a theme or a plugin, download this file and follow the steps below.

  1. In your FTP server, go to wp-includes/certificates/HERE
  2. Rename the current ca-bundle.crt to OLD_ca-bundle.crt
  3. Rename the downloaded file cacert.pem.txt to ca-bundle.crt
  4. Upload ca-bundle.crt to wp-includes/certificates/HERE

 

Source: https://wordpress.org/support/topic/ssl-error-18