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.