Your Caddyfile doesn’t seem to include any DuckDNS configuration. Because of that, Caddy is attempting to obtain its certificate via TLS-ALPN-01 challenge, but it’s failing since your Caddy instance isn’t reachable from the Internet to complete the challenge.
Try searching this forum for other DuckDNS posts—you might find something relevant about using the DuckDNS module. For example:
https://caddy.community/t/failure-to-obtain-certificate-from-issuer-with-duckdns/19411