DON'T USE ZEROSSL.COM, instead use ACME.SH to create Let's Encrypt SSL certificates. It's great!
At some point in the last 90 days zerossl.com was bought by someone who changed it from the decent site it once was. The functionality has dramatically changed and I didn't find it to my liking.
I tried using certbot and once again found it to be horrible. I tried GetSSL which was good, but didn't have a feature I needed. Then I tried acme.sh on an Ubuntu Virtual Machine running in Parallels and it worked great!
I use the Apache web server that is pre-installed on MacOS and use these SSL certificates for the development versions of Mattifesto websites.