It stands to reason that you can only send data on an open socket only if there is a socket open and associated with the packet.
This occurs only for packets opened from a logging session that has terminated but the packet list has not been closed. Packet lists that are saved and later brought up for editing, will not allow you to resend data on an open socket.
After you have opened a packet in the Packet Editor Dialog from a recently terminated logging session, you will see that by default the "On Open Socket" button is checked in the "Send" group:
With that checked hit the Play button in the actions group. It looks similar to the button which starts a logging session:
The field above the Actions group will display whether or not the operation succeeded. Additionally the field next to it may give a more specific error message:
