Give duckdns.org an IPv6 address Ask your hosting provider for the site's IPv6 address (or the hostname to point at), then publish it as an AAAA record. If your host doesn't offer IPv6, a proxy that does can sit in front. How to add an AAAA record
www.duckdns.org is an alias of appservers-duckdns-prod-1630339571.ca-central-1.elb.amazonaws.com, which has no IPv6 www.duckdns.org is a CNAME, so it can't also have its own AAAA record. Ask whoever runs appservers-duckdns-prod-1630339571.ca-central-1.elb.amazonaws.com to enable IPv6 there — or replace the alias with the A and AAAA records they give you. Why some hosts don't offer IPv6
Nameservers without IPv6 ns3.duckdns.org, ns4.duckdns.org, ns5.duckdns.org, ns6.duckdns.org, ns7.duckdns.org, ns8.duckdns.org, ns9.duckdns.org, ns1.duckdns.org, ns2.duckdns.org all have no AAAA record, so a resolver that only speaks IPv6 can't look your domain up. Most DNS providers offer IPv6 nameservers; ask yours, or move DNS to one that does.
Visitors on IPv6-only networks (many mobile carriers) can still reach it, but only through a translator their carrier runs (NAT64). The site itself has no IPv6 address — and you can't make one up from the IPv4. Why some hosts don't offer it
Copy the evidence— a citation-ready record of this measurement