Hayek says that it is impossible to plan an economy without also taking charge of all matters of supply and demand, vagaries of price, and allocations of people to professions. Hayek's proposition feels intuitively correct to me. For this reason we do projections in my company, not budgets. Planning inevitably fails, but I think that Hayek's point is that we shouldn't want to do it even if it worked. A market is a clutter of votes, thousands of votes every fraction of a second on prices, quantities and value.
Free marketplaces involve democratic choices of lifestyles, places, culture, affinities and ideas.
Unplanned free markets are an underpinning of human liberty, and have been since the first prehistoric human traded something unwanted for something desired. Hayek, in fact, seems to think of himself as a liberal, though he uses the term far differently than conservatives use it today. Hayek is usually quoted by conservatives and libertarians today, in support of their belief that Government is Bad, but Hayek actually believed that there are purposes served only by government. The fundamental problem of capitalism, which Hayek does not really address, is that powerful forces soon develop which desire to end market freedoms. The dreaded collectivism sought by socialists is eerily similar to the collectivism brought about by economic monopolies and deprivation of choice.