Are you sure there isn't something else running on port 81 on unraid?
According to your log, letsencrypt servers cannot reach your container when they try to connect to your domain on port 80, which could be that your dns is not set up correctly, or duckdns is forwarding to the wrong ip, or your router is not forwarding to the correct lan ip or port, or your container is not listening on the right port.
You can temporarily put up a regular nginx container (while letsencrypt container is down) with the same ports and try to connect to it through your domain on port 80 and see if you get the default placeholder page