Argument from noncognitivism

[Silver]

Non-cognitivism is the meta-ethical view that ethical sentences do not express propositions (i.e. statements) and thus cannot be true or false (they are not truth-apt). A noncognitivist denies the cognitivist claim that "moral judgments are capable of being objectively true, because they describe some feature of the world."[1] If moral statements cannot be true, and if one cannot know something that is not true, noncognitivism implies that moral knowledge is impossible.

https://en.wikipedia.org/wiki/Non-cognitivism