Just to clarify for others following the same path, the ca.json file is in C:\ProgramData\certify\ca.json, not C:\ProgramData\ca.json
I’m trying the same thing using step-ca (bloody internal domain from before my time using .local) and came across the same issue.
Thanks @stevo - it’s usually best to refer to our docs as forum posts are quickly out of date (and may contain inaccuracies that don’t get edited later), see Certificate Authorities | Certify The Web Docs but this raises a good point in that our current UI doesn’t let you edit the option to allow internal hostnames. This will be improved in the next release.