Hello Yorkie ! Indeed, I forgot to mentioned it in the gear configuration. Everything is set according to UWTechnics manual : Position 7 on the board for the Retra, WL is off in the camera, only mechanical shutter, power save to 10sec, etc. The following image is taken from the manual and you can see that all the flash modes are usable. In my case, the first one is grey and when try to select it, it gives me the error message. I'm starting to think it is a defective unit as the error message seems not to be encoutered by others ?