Open the command line and type this:
route print
Find the the IPv4 table.
There should be a column like the following:
Gateway
10.4.0.1
On-link
On-link
On-link
On-link
On-link
192.168.1.1
On-link
On-link
On-link
On-link
On-link
On-link
On-link
On-link
You can see that 192.168.1.1 is the IP of my router. I guess this is how fopnu looks for your gateway's IP address in order to open ports in it with UPnP. For some reason in your case there is no IP address in that column that falls within the private range of IPs.
https://en.wikipedia.org/wiki/Reserved_IP_addresses