Basically this means that your local DNS setup is preventing the SOA DNS query from resolving, so Caddy can’t determine which part of the domain is the zone (to know that nicolanapa.duckdns.org is the zone and not beta.nicolanapa.duckdns.org or duckdns.org, in your case).
You can try setting resolvers 1.1.1.1 in the tls directive (not available in global options) to have Caddy use Cloudflare’s DNS servers to resolve your domain instead of your system’s default resolver.