Hi bella243!
To your question, it's equally valid to refer to this as a Mistaken Negation or instead as a Mistaken Reversal, depending on how one initially diagrams it. Whether identified as a Mistaken Negation or a Mistaken Reversal, both demarcations identify a flaw in the reasoning given in the stimulus.
In this particular question, the author moves from the assertion, "there cannot be a good legal system where the police are not well paid," and concludes that "it follows that where the police are well paid there will be a good legal system." In other words, it starts with the following reasoning:
good legal system police well paid
Contrapositive: police well paid good legal system
Another way of saying this is that if one has a good legal system, then one knows that the police are well paid (or, if police are not well paid, then one cannot have a good legal system). From this alone, there's nothing guaranteeing that a well paid police force alone will produce a good legal system (e.g., this would also require competent lawyers and judges, access to courts, etc.). A good legal system implies well-paid police, but not vice versa. However, this mistake is precisely what the author of the stimulus concludes in saying that "it follows that where the police are well paid there will be a good legal system":
police well paid good legal system
In reaching this conclusion, the author makes a flaw in reasoning. From the perspective of my original diagram, this flaw would be referred to as a Mistaken Reversal (because it reverses the variables). From the perspective of the contrapositive, however, it would be a Mistaken Negation (because the variables are in the right spots but aren't negated when they should be). Whether one identifies this as a Mistaken Negation or a Mistaken Reversal will depend on how one diagrams the sentence "there cannot be a good legal system where the police are not well paid"--my contrapositive above, for example, might be how others wrote out the initial diagram, but they both express the same thing.