Marx was wrong because he predicted a proletarian mass movement to arise from highly developed capitalist relations, but this never happened, In fact, there has never been a proletarian revolution in all of history. The bolsheviks got into power through a coup and the masses were mobilised by the second world war going badly and not by capitalist relations they had whished to be destroyed. In China, Vietnam, Korea and Cuba we see mostly national liberation struggles being the cause of a regime change, it just so happened that the forces winning out in the end were the communist parties. Lastly, there are countries like the GDR that did not only not have a proletarian revolution, they didnt have any revolution but simply lost a war against socialist forces and was declared socialist afterwards. So in no instance did socialist rule establish itself through a mass movement that had the goal of dismantling capitalism.
The reason for this is that class counsciosness doesnt exist or at least doesnt exist in a historically or politcially relevant manner. Marx predicted that class counsciosness would arise in those situations were capitalism is developed the strongest and the internal contradictions of capitalism create the highest pressure upon the working class, instead we have only seen socialist attemps in places were capitalism was poorly developed and the centers of capitalist production even seem to be the most stable.
Other points worthy of critique in marxism are it's conception of history (communism is is no historical destination), his disregard of the states own reproduction of hierarchy and his own petite-bourgeois prejudices against the lumpenproletariat, but even I don't fully reject marx. I think his theory of value and exploitation is spot on and alot of his sociological observations are REALLY good, but there just wont ever be a class counscious mass movement fighting to emancipate themselves from capitalism. That's just a pipedream.