Hurray - I have solved by problem.
I had installed the latest driver for Mac from Prolific, and it seemed to have installed OK, but I couldn’t see any sign of it I still can’t.
I had tried an Apple cable and an Anker cable, switching the plugs through 180 degrees in all combinations - no luck. Then I tried a cable from GoPro - it was a USB-A to USB-C. So I added an adapter to make it USB-C to USB-C and bingo - the Mac reported a new device.
The connection from the Serial Config Tool then connected first to a bridge and then to the emonTx4.
I’ve changed the input sensor types now - all is good.
Phew ….