True communism in a country is impossible.

You can have socialism, or anarchy, which we've seen before, but communism cannot function in one country alone, unless said country is completely and absolutely self reliant.

A major part of communism is internationalism, which is why socialist countries had the Comintern. (Communist International). Besides a political/social system, communism has a strong basis as an economic system. You can't apply communist economic system principles to the capitalist market.

To my knowledge, no existing country is self reliant to the point that they can completely cut off trade with the rest of the world. USSR didn't do it, China didn't do it and they were the two biggest countries at the time.

That, of course is all a very surface level ELI5, and if you want to ask something more specific or in depth, feel free to.