For issues with macOS Catalina, check https://github.com/balena-io/etcher/issues/2833 and https://github.com/balena-io/etcher/issues/2911 for some temporary workarounds. We’re working on a PR that you can check here that will solve the new OS issues.
As for the ISO, as mentioned we would need the console logs to investigate further. If it’s zipped, try extracting it if you want to give it another try as there are some ISOs that might not be correctly recognized when inside an archive.