The reason (as pointed out by zeraholladay) is that nmap scans per default only about ~1000 ports. If the service is running on a non standard port it will not show up in the default scan
Noooo! Don't give away the keys to the kingdom. :)
Instead, maybe say "no ping scan".
On the other hand, if you're aware of nmap and know how to use it to get past the first hurdle, you're probably more than capable of solving the actual challenge.
