Marx doesn't say how communism could be achieved. It's an ideal. At the end of history communism will be achieved.

So if you think it's possible, you have to help us figure out how to build it.

The core idea of communism is the goal for a classless, stateless society where all are equal. Unfortunately for us, the Soviets have successfully made the concept associated with Bolshevism / Marxist-Leninism, which calls for a vanguard party to represent the working people, i.e. an authoritarian one-party state. Communism by itself is not the antithesis to democracy, but Marxist-Leninist states are usually very authoritarian. And once you have a dictatorship, you foster a cult of personality and policy is based on the decision of one person, and that never ends well for anybody but them and their inner circle.

Communism was born when capitalism was uncontrolled, working people did not have minimum wages with poor working conditions and, often times, no rights either if they were the wrong gender, background, religion, sexual orientation, or simply social class. Don't get me wrong, capitalism now is still bad (for both people and the environment), but unions are stronger, restrictions on corporations is stronger, and society is more equal. Freedom of speech and the press is more prevalent, though certainly more can be done, and inequality of all kinds is still a global issue that must be tackled.

Is communism "dead" then? No, of course not. I respect all who work towards the goal of an equal, classless society freed from the shackles of capitalism. But those who want to stifle democracy and establish dictatorships, I do not respect.

My extremely uninformed and non-professional take is that communism only really works on small scales. at a small scale, maybe less than 150 people, what actually needs to be produced can be handled by a few people who understand the needs of the community, or people can present what their own needs are and others can help them. For incentives, pretty much the only one is dopamine like you said, but helping out in a community and seeing the impact should not be underestimated as a motivator, and neither should reciprocity (I got helped so I'll help other people).

Larger than that you'll need an organizational structure to handle needs and potentially an enforcement structure to handle motivation or lack thereof, inherently leading to people with more power than others who might abuse that power to garner more of it.