Here is that quote on the false solution to the problem of exchange put forward by marginal utility theory. It’s from the reification section of History and Class Consciousness (1923):

“Marx acutely summed up this situation with reference to economics when he declared that “use-value as such lies outside the sphere of investigation of political economy.” It would be a mistake to suppose that certain analytical devices – such as we find in the ‘Theory of Marginal Utility’-might show the way out of this impasse. It is possible to set aside objective laws governing the production and movement of commodities which regulate the market and ‘subjective’ modes of behaviour on it and to make the attempt to start from ‘subjective’ behaviour on the market. But this simply shifts the question from the main issue to more and more derivative and reified stages without negating the formalism of the method and the elimination from the outset of the concrete material underlying it. The formal act of exchange which constitutes the basic fact for the theory of marginal utility likewise suppresses use-value as use-value and establishes a relation of concrete equality between concretely unequal and indeed incomparable objects. It is this that creates impasse.” [Emphasis added]