Port forwarding on Airport Express IPv4
IPv4 port mapping will only be available if the AirPort Express has its router mode set to DHCP and NAT. You can check router mode setting in Network tab of AirPort Utility.

If you happen to have the router mode set to one of the other settings, then you may have another device on your network running NAT and port mappings would need to be configured on that other device. Another option would be do disable NAT on other network devices and enable it on the AirPort Express so that you can manage port mappings on the AirPort.