Did Karl Marx believe in the class struggle between the rich and the poor?

I don't know what you mean by "believe in it". Marx believed there was such a struggle and the rich always won. The central concept of Marxism is that the rich control the means of production (they own the mines, the factories, the land, the distribution networks) which should really be owned by the people as a whole. The idea is "Give what you can and take what you need". The flaw is that there is no reward for working hard or being smart. Joe sleeps until noon every day and lets his crops rot. Charlie rises early and grows bumper crops. But in the communist system they both get the same reward.

There is not and has never been a true communist state anywhere. There have been dictatorships that call themselves communist. Vladimir Putin is believed to be one of the richest men in the world,