There's a lot of different versions of this claim so it's hard to engage with. One that basically points out that Marx defined Communism specifically not as the ideological governmental system established by the overthrow of capitalism, but as the stateless organization of society which would be organized as the end result of socialist revolution in the long term, after the victory of international revolution. But there's also the Left Communist version which says that really, Leninism betrayed some fundamental principal or another or what socialist/communist revolution would be and thus all later development within the USSR are stained by that original betrayal (usually declared as Lenin destroying the democratic character of the Soviets, or Lenin not turning the Russian economy into a series of worker managed cooperatives). There's lots of different "the Bolsheviks don't count" schools of thought for communism. Which doesn't even touch on how the anarchists sort of get involved here.

And as for truth? There is no real truth about this. It's a matter of defining Marxist orthodoxy and whose interpretation of Marx is right. Speaking personally I would say the pedantic point is correct. They weren't communist because it's a misnomer to say that a state is communist. But I would say they were a certain kind of socialism. But that they're not the only possible kind of socialism. The way this is commonly stated by the left communists is correct in that heavy handed state control and dictatorship isn't actually fundamental for socialism. But they're wrong when they say definitively that then it isn't socialism. I would also dismiss the claim this is the only way revolutionary socialism can turn out.

What we know of colloquially as "communist states" did not themselves claim to have achieved communism. It was a term used to mean states that claimed to follow Marxist or Marxist Leninist thought. The USSR would be "building socialism" and "building communism" as their political / ideological goal, the problem was it seemed to be ever receding into the distance. There is actually a Soviet-era joke about this, in the popular "Radio Yerevan" QA format:

Radio Yerevan was asked: "We are told that the communism is already seen at the horizon. What then is a horizon?"

Radio Yerevan answered: "Horizon is an imaginary line which moves away each time you approach it."

If you want a primary source here for this distinction between "the communist states / countries" (that historically existed) and "communism" (the idealised end goal of these states), you can take a look at the speeches of the 22nd Party Congress. This was in 1961, and in it Khrushchev made the following bold pronouncement:

"Мы руководствуемся строго научными расчетами. А расчеты показывают, что за 20 годы мы построим в основном коммунистическое общество"

A rough English translation would be "We're adhering to strict scientific calculations. They tell us that in 20 years, communism will be mostly achieved".

Now, I would agree with the statement as outlined in the OP that "real communism has never existed", under this strict reading - that the idealised end goal of a communist society has never been achieved. But that's not necessarily the same as saying that a country has never strived towards it, and thus we can't know anything about the difficulties faced or how practical / impractical it is.

When most people make the statement that "real communism has never been attempted", they are usually not splitting hairs in the same way that I am. Rather they are taking issue with the path taken by historically communist countries on the road to communism. Stalin and Lenin's impact was huge on subsequent revolutions and some see this as a kind of corruption of the ideology as a whole. Eliphas8's post goes into this too.

No real communism has never existed.

The ill informed will often point to the former soviet block and china as examples of communism. This is however a farce, as these states are in fact examples of state capitalism.

Communism entails the abolition of capitalist labor relations (private property and wage labor). The abolition of the capitalist state and the creation of the 'political' dictatorship of the proletariat, political referring to the social aspects of society.

The closest humanity has come to communism can be seen in tribal society's, where the good of the community is put first and private property does not exist.

True Communism has existed on many occasions. Many tribal societies could be described as communist (mostly the ones on the fringes of larger civilizations). Inspired by Diogenes of Sinope a huge number of communes were built in the ancient mediterranean.

In the modern era it existed in large scale in the paris commune, the spanish revolution (in anarchist Aragon and revolutionary Catalonia), parts of russia like Kronstadt before the Bolsheviks crushed it, the Strandzha Commune in Turkey, the Ukrainian Free Territory, the Shinmin autonomous region in Korea, and the Shanghai People's Commune.

It has existed on smaller scales in any intentional communities. Notable examples are Freetown Christiania and the Life and Labor Commune.

It also exists in Zapatista territory and looks to be forming in Rojava.