Neither of those countries ever claimed to have a communist mode of production. Being led by a Communist Party and having a communist economic system are two different things. The USSR never claimed to have achieved communism, they achieved socialism (a transitionary phase between capitalism and communism) according to Marxist-Leninist theory.

This is a quote from Engels describing such a transitionary system:

What we have to deal with here is a communist society, not as it has developed on its own foundations, but, on the contrary, just as it emerges from capitalist society; which is thus in every respect, economically, morally, and intellectually, still stamped with the birthmarks of the old society from whose womb it emerges.

If an advanced capitalist country in the imperial Core, today, attempted a similar transition, it would be nothing like these other examples because our material conditions are entirely different (Marx didn't expect communism to be tried within individual countries in isolation or within undeveloped countries). Marx didn't provide a blueprint for transitioning to communism because what that looks like is different in every country and material situation etc.