Do you have a CNAME record from _acme-challenge.cnamed.example.com to target.duckdns.org as explained in the README?
Do you see the TXT entry being changed on your DuckDNS domain when Caddy is trying to solve the DNS challenge?
You can run dig target.duckdns.org TXT to check if the records are there.
You could also make a request to https://www.duckdns.org/update?domains=target.duckdns.org&token=<token>&verbose=true to check as well.