I am using Airport Express to connect and access the net, I have 6 devises ( 1 pc,1 Mc, 1 iPad, 1tablet, 1 cellphone and 1 smart tv) 5 of the access the net wirelessly via Airport Express and the tv is ethernet connected to Airport Express with a cord using the ethernet outlet in the back of the Airport Express, how did you get to the last part of your screen shot that says port Mapping Set up Assistant.
If you want to set up Port Mapping on the AirPort Express.....then it must NOT be configured to operate in Bridge Mode.
To see if you have set up the AirPort Express in Bridge Mode......which will not allow Port Mapping on the AirPort Express......
Open AirPort Utility
Click on the AirPort Express
Click Manual Setup
Click the Internet globe at the top of the window
Check the setting for Connection Sharing. If it is currently set to "Off (Bridge Mode)", then you must set up Port Mapping or Port Forwarding on another device.....the one that is acting as your modem.
That is why we first asked what type of modem or connection that you were using to provide your Internet service.