The command certbot --nginx will take an unencrypted website on an Nginx configuration file, get a certificate for it and change the configuration to use that certificate and thus HTTPS. Sometimes, however, you are given an Nginx configuration template that already has encryption/HTTPS, so running the automated certbot --nginx is not possible, as it will simply give an error saying that the certicate that Nginx is looking for doesn't already exist and thus the Nginx config is broken.
