Not sure why this comment is downvoted. In the context of Marxian Socialism and Command Economies, it's very much true that socialism is incompatible with free markets.

Going even further, without a free market for labour, there's no true worker emancipation. Even with "worker-ownership" (read corporate-ownership under a democratic governance), each worker's power is still diluted by the governing body (which, funnily enough, means socialism strengthens capitalism as "control by those who own capital" while weakening capitalism as "free markets").