The first challenge was that no green LED’s lit up on either the motherboard, the stepper cards nor the extruder.
All I could see was a red flashing light.

In short, don’t get confused by the error message. The power to turn on is not at the extruder, but at the motherboard.
There is a power switch on the lower left part of the motherboard – this should be turned on.

If you are getting this error when the power is turned on there, then something else is the issue 🙁

When all works well you will get a message about the motherboard version and extruder version in the text box below the main window (where the build area and model is shown)