Hayek on Keynes' First Fundamental Error: Keynes' General Theory Lacks a Theory of Capital and It Suppresses the Production Structure in the Concept of Aggregate Investment

From Hayek’s point of view, the major deficiency in The General Theory is that it is not based on a theory of capital. According to Hayek, the market is a network of millions of companies that complement and coordinate with each other intertemporally and synchronically, forming an extremely complex production structure. In order to understand how and why this structure is coordinated or discoordinated, we need to apply a theory allowing us to study the way it works. However, Keynes does not study this production structure, but suppresses it in the concept of aggregate investment. This is why Hayek thought that Keynes was not able to understand the causes of and the solutions to economic fluctuations.

--David Sanz Bas, "Hayek's Critique of The General Theory: A New View of the Debate between Hayek and Keynes," Quarterly Journal of Austrian Economics 14, no. 3 (Fall 2011): 294.