It means you haven't port forwarded. You can't receive inbound connections without Fopnu using NAT traversal. Fopnu can automatically port forward for you if your router supports it. (almost every router does)
There's two reasons why Fopnu was not able to automatically port forward:
1) Your router doesn't support UPnP / NAT-PMP. If this is the case you can manually port forward if you give your computer a static (LAN) IP. Some routers support DHCP reservations for this. After you setup the DHCP reservation or static IP, you can create the port forwarding rule from your router's webUI. If you don't know how to access your router's webUI, open command prompt or powershell and run the "ipconfig" command. Enter the default gateway IP in your browser. Common examples are
"http://192.168.2.1/" and "http://192.168.1.1/"
. For the port forwarding rule, keep it as a 1:1 (one to one) rule. What I mean by that is if UDP port 43857 is the port your Fopnu uses, then the rule would use that for this port on router's public IP and LAN side. Example: WAN UDP port 43857 to LAN 192.168.2.250 UDP 43857. Your port number and LAN IP will almost certainly be something different. This is just an example. I'll send a screenshot example later.
2) Your ISP uses something called carrier grade NAT (CGNAT). It's also known as large scale NAT or NAT444. You'll have to ask your ISP about this. You can't port forward from your router so you'll have to ignore this or use a VPN. I also recommend checking to see if your ISP supports IPv6. You can go to https://ipv6-test.com/
If your ISP supports IPv6, you do not have to configure port forwarding for IPv6. IPv6 doesn't use NAT or CGNAT like IPv4.
You can still use Fopnu without port forwarding but if you are able to it's always recommended to do so.