Is the NXDOMAIN error the only error? Usually LE errors dump a lot more info. I’m assuming you’re attempting a wildcard certificate, otherwise LE would first try to verify your domain using http before DNS.
Wherever your DNS is hosted, several of your domain’s host names (ns1., www., mail. etc.) need an A record pointing to your server’s IPv4 address. AAAA records are for IPv6.
How “fresh” is the setup? If it was setup today and DNS is configured properly there probably hasn’t been enough propagation time for LE to find your server. In that case just wait and try again later.
If you are hosting your own DNS with Webmin’s BIND server, all necessary A and AAAA records should be created automatically when a virtualhost is created by Virtualmin. You can view and edit a domain’s DNS setup from Virtualmin’s DNS Records menu.
If instead of Webmin’s BIND you host DNS with a domain registrar or DNS service, you’ll need to setup A records manually in their control panel.
LE is bombing for some other reason if DNS is already setup and resolving. Often the problem producing this particular error will be network related but not necessarily on your server or network. When you spot an ACME IP address in your LE error messages try pinging the IP or better yet, see if you receive all hops when running traceroute on the IP. If traceroute stalls with LE but finishes for other IPs it’s possible the LE side is the problem and is likely temporary.