I removed the tag "lie" and added the tag "false". While the sentence is definitely false by every possible standard (even the USSR never claimed to be a communist country), people who say it aren't necessarily being intentionally deceptive.

Union of Soviet Socialist Republics. Socialism is the transition step to reach communism under communist ideology, so it's part of the communist system. I think you're confounding "communism" the state of being with the ideology, which they did have. It's fair to call them communist under either.

The party was called "Коммунистическая партия Советского Союза" (Communist Party of The Soviet Union), and all of them called themselves communist. That's just flat out 100% wrong

It's not fair to say that communism was the system practiced in the Soviet Union. It's flat out 100% wrong, and nothing you just said makes any difference to that.

Under communist ideology, communism is a classless, stateless society. None of the Communist Party leaders claimed that they had built such a thing, nor did any outside observer believe they had. There is no one who has ever made the argument that the USSR was a classless, stateless society. If it was, how could anyone oppose that? But it wasn't, and therein lay the problem.

They said "our children will live to see communism" because the leaders of the USSR knew better than to say "we are living under communism." Because they weren't, and they never claimed to be, and no one ever claimed they were.

The term "communist" covers communist ideology, otherwise it wouldn't be called "communist ideology". It actually does so much more commonly than the way you mean it by, which is almost entirely just by communists, and generally just selectively to avoid this kind of thing.

It requires either ignorance or a deliberate misrepresentation of the word "communism", which was defined long before the USSR came to be, to claim that "communism is the system practiced in the Soviet Union." It makes even less sense than saying that "democracy is the system practiced in the Democratic People's Republic of Korea" - at least the DPRK claims to be democratic, unlike the USSR which never claimed to be communist.

Even if the USSR *had* said it was communist, it would still not be true that they were practicing something that was defined before they ever came into the picture. But in fact, they didn't claim to be a communist society. No one would have accepted it, back then, if they had said they were living under communism, because communism was something that was widely desired and that they were nowhere close to. If they could've gotten away with it, they probably would have claimed to have achieved communism.