Perhaps explain that the constructor should throw an exception when the argument has got a missing pair e.g 'ABC'.

It has an explaination about having a maximum of 10 pairs and to have unique pairs. I think it should say that a valid setup should not have half a pair.

Each pair of letters is a bidirectional mapping, the first two letters 'AB' mean A -> B, B -> A, and so on. How would you explain it differently?

Should not have accepted a partial wire definition

What is it actually expecting? It is not explicitly described on what needs to be implemented

The Plugboard class you will implement, will:

Take a list of wire pairs at construction in the form of a string, with a default behaviour of no wires configured. E.g. "ABCD" would wire A <-> B and C <-> D.
Validate that the wire pairings are legitimate. Raise an exception if not.
Implement the process method to translate a single character input into an output.