No IPv6 (AAAA) address
DNS did not return an AAAA record for the host.
Why it matters
Clients that only have IPv6, and dual-stack clients that prefer it, cannot reach this host that way.
What to do
Publish AAAA records and serve HTTPS on those addresses if you want IPv6.
Reference
Modern public sites should be reachable over IPv6 with the same TLS guarantees as IPv4.