locked IO error when testing PTT control with 'no rig'


I am new to WSJT-X and just getting it running on an Ubuntu laptop into an FT-101E. It seems to work and I have had a few QSOs on 20 meters.
My question is how do I set up the PTT functions in the settings? It currently works with the FT101E using the VOX mode. However, I want to use WSJT-X on 23cms with an FT-736R and need to trigger the PTT for correct sequencing.
At the moment when I attempt to set the PTT options to use the serial port the test procedure gives me a message "Rig failure Hamlib error: IO error while opening connection to rig".
The Radio settings I am using are 'no rig' and the interface I am using is a UD06YA (a USB interface that uses a PL-2303HX chip that controls the RTS line of the computer serial port). Ubuntu recognises the interface as /dev/ttyUSB0 and that shows up in the Port field of the settings, so there doesn't seem to be a problem there.
What does the IO error mean? Is there a config file somewhere that needs changing?
George, vk3eip

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