Nslookup will try to resolve the name for the ip address of the DNS server configured as the primary DNS server on the client by performing a reverse lookup of the ip address. If you don’t have a rDNS zone set up for your network/subnet you’ll get the “server unknown” message as nslookup will be unable to resolve the name for the ip address.
It’s not an error condition and won’t cause any problems for normal AD and DNS operations.