The "theory" of communism or what people try to argue for is predicated on the idea that the labor of a worker automatically produces surplus value which belongs to them, but is taken as profit by the capitalist.
In reality, the worker only adds their labor to the value of any product. Whatever other value is derived of the product is no longer the business of the worker, but others who extend the value of the product - for example, a blacksmith who makes an axe has nothing to do with the value created by using said axe so there's nothing extra they're owed beyond the necessary amount that is needed to sustain them.
The criticism by Marx was based on the idea that the capitalist artificially and needlessly inflates the price in between the production and the actual use of the product and de-stabilize the whole system. He was not entirely wrong. Marx actually commended capitalists for bringing about all the technological and economic advances that are necessary for socialism to succeed. Without capitalism there cannot be socialism; the practical communism and socialism only fails in the assumption that after capitalism history becomes completed to the point that capitalism is no longer needed. Then they rushed the point well before capitalism had provided them the necessary means to succeed, such as in Soviet Russia which went straight from feudalism to communism without developing capitalism in between.
In reality, Marx was a Hegelian and recognized that after capitalism comes socialism, and after socialism comes capitalism… in a never-ending cycle.