A private, inaccessible, IANA/IETF-reserved IP address was found for servmin.duckdns.org. Let's Encrypt will always fail HTTP validation for any domain that is pointing to an address that is not routable on the internet. You should either remove this address and replace it with a public one or use the DNS validation method instead.
2002:5b61:711f:8000:3631:c4ff:fe65:b8e3
A and AAAA records found for this domain
servmin.duckdns.org. 0 IN A 91.97.112.167
servmin.duckdns.org. 0 IN AAAA 2002:5b61:711f:8000:3631:c4ff:fe65:b8e3