To use the camera in team mode the ST-16 needs to be bound to a second device. Team means more than one and if all you have is the ST-16 with nothing else to link/bind to it will revert to single mode.
On a side note, if performing a system upgrade and the system does not recognize the camera/autopilot upgrade, put the ST-16 switch in the team mode and go back to system settings to refresh the firmware. Team mode puts the -16 in a "search" mode where it is "looking" for something, which seems to make it easier for it to locate "flightmode" firmware. When the firmware upgrade is complete you'll find the -16 has gone back into single mode.