§188

At the end of §187 Wittgenstein asked what was wrong with the interlocutor's (A's) idea. What was the interlocutor's idea? - That in meaning the order A had already taken all the various steps- "In meaning it your mind, as it were, flew ahead and took all the steps before you physically arrived at this or that one" - meaning something anticipates reality.

In §188, as far as I can tell, Wittgenstein does not say what was wrong with the interlocutor's idea - he just spells out what the idea was.

In 'Accord with a rule' - in the analytical commentary vol.2 - Hacker compares the problem here with 'the mystery of negation'. The problem is that (iii) seems to conflict with (i) and (ii) in the following triad of propositions:
(i) What a true proposition depicts is what is the case.
(ii) What a false proposition depicts is not what is the case.
(iii) What a proposition depicts is the same, no matter whether it is true or false.

In the Tractatus Wittgenstein had 'solved' this mystery by saying that what a proposition depicts is a state of affairs - which is a possibility, not an actuality. States of affairs mediate between propositions and facts.

Similarly in the case of following a rule meaning the steps in the extension of the series is mediate between the rule and the applications of it - it is what connects the two (this is the confused view of the interlocutor).