Thanks leader for your reply. Having been thoroughly misled by the Juniper CSR documentation, the error was nearly a complete mystery (there is simply no indication that the CN field is critical to the ca-cert verification and certification process–as indicated, the info.ssl.com was extremely helpful. You might even provide a link on your page to their site.
At your suggestion, I was able do download the ca-cert from the crt.sh website and proceed to load and activate the letsencrypt.org ssl certificate so 
joy has occurred at long last (the activation procedures in JunOS were not easy, again due to Juniper documentation deficiencies).
I did report the problem to the zerossl.com support site and they have already followed up. They admitted that it was an unusual and perplexing response being that everything else was successful so they are already working on it. Thanks again to everyone who contributed.