This won't be a very popular response, but: generally the answer to this has been to question the framing of authoritarianism. Marx spoke of a society in which

This won't be a very popular response, but: generally the answer to this has been to question the framing of authoritarianism. Marx spoke of a society in which labour is no longer alienated, but "man's prime want". His assumption (true or false) was that people wouldn't be need to be pressured to work. His claim was that in capitalism, itself an authoritarian system descended from previous class systems, authority lurks behind free rights and duties, in which he commented that under capitalism the worker must be free - free to sell his labour, and free from the means of production. In summary, to anti-capitalists the question is not only how to have an economy without authoritarianism, but also how to rework the very concept of labour, how it is thought of, practised, etc. What is it about labour in class societies, they ask, which means that it can only be seen as something which needs to be compelled, either by holding the means of survival (a wage) over the worker's head, or by placing them in a state-industry job for labour tokens? Both the capitalist and 'socialist' solutions are authoritarian. Why? Unfortunately a lot of the popular discourse around Marx's actual works has been obscured or discounted. Yet it is impossible to validate the USSR, Cuba, China, etc. as 'Marxist' or not without knowing what he wrote himself. It's just as rare to find a leftist who's read Marx as it is to find a right-libertarian who's read Marx.