A good argument can be made that the Communist economic system failed pretty badly. The capitalist economic system fails pretty badly too in many places, but it fails in a different way, and it seems that many folks prefer a failed capitalist system (as we have in the US right now and in my other countries) to a failed Communist system.
The system that failed was one where the state owned the entire means of production. The economy was a planned economy, often referred to as the Command Economy. It didn’t work very well.
The USSR had something called Gosplan, a huge building in
Moscow that housed that part of the state that planned the economy every year.
For instance, Gosplan would have to decide how many eggs to produce per person per year. They would have to make a wildassed guess of how many eggs the average person would eat in a year, and then they would produce that many eggs. It is almost impossible to plan an economy this way due to the vicissitudes of human nature. Also not pricing things by supply and demand caused all sorts of problems. The prices of things often did not reflect their true cost, so things would be sold for far less than it cost to produce them. This pricing problem caused all manner of problems.
Needless to say, with the collapse of the USSR in 1991 and the subsequent fall of the East Bloc, this system was rejected as a poor model for an economic system.
So when people say “failed socialism” they are referring to the Command Economy like the one that the USSR had. This is the only type of socialism that has failed. There are many other types of socialism, and they still all work just fine.
However, rightwing propagandists and liars have seized on the failure of the Soviet system to wage a disgusting and dishonest campaign against all types of socialism, including those that work just fine. Lately it has been expanded even to social liberalism, which is the leftwing economic model that the US has, since we really do not have a social democracy. I suppose social liberalism can be though of as social democracy light.
They have also expanded “failed socialism” to include all government regulations of business and the entire welfare state including Social Security and Medicare. Anything the government does other than cops or the army is “failed socialism.” What they are really saying is that Social Security and the other welfare state programs and environmental and all other regulations are all Communism.
The right wing has actually been saying this since the end of World War 2 in this country when the Cold War started up. The US government got in on this lie, and the US state has been propagandizing the “failed socialism” model every time some leftwing government shows up in Latin America and tries to raise the minimum wage.