humans are greedy and selfish

That’s just what you choose to believe. These’s no scientific proof that they make up some kind of transcendent truth about the human condition.

It’s entirely possible that the humans traits of greed and selfishness tend to become much more pronounced when humans are subjected to systems that reward those qualities.

Also, greed and selfishness are distinct from self-interest. And besides, self-interest doesn’t explain the entirety of human behaviour either.

Perhaps other systems that reward collaboration and egalitarianism and autonomy are not only possible, but also more sustainable that the shitshow we’ve got now, and all we need is for enough people to get out of the mental rut of believing capitalist bullshit about “humanity” and “life” just because it’s the status quo.

The state is needed to enforce private property, not the lack of it.

Critically, you assume that authority is required for order, so obviously you’re unable to envision whatever it might be that anarchists are getting at.

Sure, communism is an ideal that does not exist in practice currently, but the point of an ideal is to work towards it. The “anarcho” part specifies complete opposition to authority in praxis as well. Strategies could include unionising, community building, mutual aid, permablitzing, FOSS, copyleft, and whatever else can undermine the current power structures while maintaining anarchist principles. Which explicitly excludes Marxist-Leninist strategies of coopting the state, or forming any other kind of heirarchy.

Name a country where communism was adopted in the first place

I can’t provide you with examples if I don’t know what you think communism means.