It's never been achieved because it was never really tried. To reiterate, what USSR and China had was not communism even according to themselves - I'm not even talking about impartial outside observers here! - so, pray tell, how is it the "no true Scotsman" fallacy?

Both China and the USSR conducted mass collectivation, land reform, commune models among other things to move away from capitalism to a communist model of public ownership. They even killed dissidents, violently oppressed public protests and starved their own people in the attempt.

And it was a massive failure.

It's a "no true Scotsman" fallacy because as long as attempts at moving to a communism model fail, supporters will just say "that wasn't true communism" ignoring the fact that if you can't actually create a functioning communist system that's indicative of failure of the model itself.

Like I said, everything that you have describe was not communism - it was a particular recipe for supposedly building it. And yes, that recipe was a massive failure, and few people on the left believe that it could have ever been anything but.

But here we're talking about communism naturally arising out of technological post-scarcity. What does that have to do with Soviet model of "public" (actually, state) ownership, or killing dissidents, or violently oppressing anyone?