The software is definitely doing the wrong thing in attempting to get a certificate for www.duckdns.org, which is not a domain name that you control and which you won’t be able to get a certificate for. But when it also tries to get a certificate for your own name, I’m not sure why it doesn’t work. One potential interpretation would be about port forwarding, but it looks like you have indeed tried to take care of that in terms of forwarding port 443 to the appropriate container, so that’s not necessarily the explanation.
Do you have a way to get in touch with people who were responsible for creating this container and asking them for support? That might be the easiest approach since I don’t think anyone on this forum has used this particular container or is familiar with how it’s set up. We can in principle try to help you debug it, but if you can contact the original developer, you might get a better and more straightforward answer.