Richard M. Ebeling, in an article entitled Austrian Macroeconomics: Review of Roger W. Garrison's Time and Money, writes a very concise definition of "the role of prices." I want to put this up because I think it clearly explains why communism is impossible, i.e., why it is impossible to eliminate prices.
The role of prices under conditions of imperfect knowledge in markets with continual change is precisely to have an institutional mechanism that enables actors to coordinate their activities WITHOUT their needing to possess ALL "the data" of the market as a whole. Market actors can use their special and localized knowledge of time and place in the division of labor WITHOUT ANY OF THEM POSSESSING THE GLOBAL KNOWLEDGE OF THE ENTIRE ECONOMIC SYSTEM. (emphasis mine)