This is the go-to copy pasta when people say nonsense like this.

Warning: lots of learns and I'm probably going to get banned

Marx's Law of Value is much misunderstood by mainstream economists (and often even by self-proclaimed Marxists themselves). Usually when a mainstream economist who did not go out of their way to study Marx with some depth argues against "Marx's labor theory of value" they are arguing against a strawman of Ricardo's labor theory of value, not Marx.

One huge source of confusion is that economists often think that the Law of Value is supposed to be a competing explanation of the same "thing" that their own Marginalist/Subjective theory of value is, and that the two are mutually exclusive. That is not the case. Marginalism is a microeconomic theory about individual, subjective preferences that deduces the familiar supply/demand curve that we know rules the behavior of prices. The Law of Value is a macroeconomic theory about the inner logic of capitalist production as a whole and how it distributes and disciplines the total pool of social-labor time available to "the economy".

In this theory, "Value" (defined as the abstract labor-time that is socially necessary to reproduce a commodity) is a theoretical construct that serves to link several different economic phenomena (kinda like how "Energy" and "Entropy" are theoretical constructs that link several different physical phenomena), not a description of what individuals in a capitalist society find valuable or not like what the Subjective theory of value does. With out properly studying the whole point of this theoretical construct, mainstream economists very rarely engage with the theory in it's own terms.

That's not to say some mainstream economists have not engaged with the Marxist paradigm fairly. In the early 20th century the economist Bortkiewicz tried to mathematically model Marx's Law of Value as set up in Vol. III of Das Kapital and wrote an important critique of it, which has led to an enormous series of responses and new developments by Marxist theorists in response. Recently Mark Blaug wrote an empirical appraisal of Marxist theories, to which the Marxist Fred Moseley wrote an amazing reply1. About a decade ago the semi-mainstream semi-heterodox economist Deirdre McCloskey (perhaps one of the most sophisticated economists around) wrote a positive appraisal2 of the empirical work developed by many Marxists, particularly of the economic historiography written by Paul Sweezy which she takes as a 'standard' of great empirical work. And then there's people like G.A Cohen, Herb Gintis and Yanis Varoufakis which are generally respected by economists all-around.

Tl;dr: Marxian economics are not entirely in opposition or even "wrong" in terms of their relation to main stream economic theory.

1. https://www.mtholyoke.edu/%7Efmoseley/working%20papers/BLAUG.pdf

2. http://www.deirdremccloskey.com/editorials/marx.php