OP didn’t respond so I will take a shot. My understanding is that under communism, the economy has to be planned by the government. Under capitalism, the price of shoes (for example) is usually determined by the demand for shoes and the amount of shoes that can be manufactured. If demand falls, the price falls. If manufacturing capacity increases, the price falls more, etc. This mechanism has feedback loops that make it efficient. In theory, companies never make more shoes than they can sell, because if they do make too many, they can sell the excess by cutting prices. Under communism there is no free market, so the mechanics of supply and demand don’t work. Some communist bureaucrat conducts a study and estimates that the country will need 100k pairs of shoes next quarter. The government then makes those shoes in a state-owned factory. Suppose, though, that it turns out that the country needed more. With no free market, there is no competitor to step in and meet the demand for shoes - now you have a shortage. Similarly, you can have considerable waste if you grow too many apples or whatever. In true communism, there is no price to adjust - you either have an apple voucher or you don’t. Thus there is generally more problems meeting demand efficiently. This is, in fact, exactly what we saw under the Soviet Union - the stores were often stuffed with unwanted items while long lines developed for items that were in high demand. Without any consideration of authoritarianism etc., this is an often-cited reason for the failure of communism.

I am not a political theorist or an economist, so please correct me if I am wrong.

They’re almost certainly conflating communism with the authoritarian flavor so enjoyed by the tankies of .ml/hex/grad