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)