If you are using the plugin WP Encrypt and when registering for the let's encrypt server to generate your certificate, you are getting this error.
Easy fix. Go to your plugin directory with FTP or file manager and find the Wp-Encrypt plugin folder. Then go to the inc > WPENC > Core
Find the CertificateManager.php
file. Edit it with a code editor and comment out the bellow section:
Now tray again to register. Wala... regenerate the certificate and upload it via cPanel to renew your Free Let's encrypt ssl.