It is incorrect to say that there was/is communism in the country. There are only countries that trying/tried building communism.

In every other country ruled by communists saying they are building communism, banning every other political party then the communist one, killing people in the name of communism

Having a single party is the one of several approaches. But the only one that survived. I think the idea is that you don't need other parties if you building communism (like why do you need any liberal party driven by rich people?). And It's not like regular people can't join this party (unlike now when only rich people can be in politics).

proof of that it’s not really possible.

I’m not saying that it’s possible, but I wouldn’t say that it’s impossible given a certain level of technology.