Postmodernism rejects positivism because positivism believes in the possibility of knowing objective truths about human societies. It seeks to find universal laws that govern societies. Postmodernism rejects the idea that such laws can exist and even the idea that any sort of objective truths about society can be discerned.
Postmodernism believes, for example, that there is no such thing as an objective reality in the human world. People only know reality through their own perceptions. Their own perceptions are, of course, mediated through many things including the language they speak and the values and attitudes that they have learned.
Postmodernism, then, rejects positivism and its certainties. It argues, instead, that we should look at things like the way that gender and race are socially constructed instead of trying to take these things as objective realities.