Hi. Depends about what is the research topic and what scientific disciplne it is. Usually Introduction, and then Methods -  because selection of "standpoint epistemology" has a lot to do with design of the whole research. And what are advantages of postpoitivist approach should be expressed in comparison to other existing approaches. A way You start Your research, has impact on its design and interpretation of research findings. Good luck with Your research.

Thank you for your answer the scientific discipline is management sciences, and the research is conducting on the Human resource management . And the same procedures are following that one follows while writing a research paper, making a theoretical framework, building hypothesis, creating objectives, writing literature and testing the relation.. So how could i justify that among applying all this processes i have applied a post positivism approach 

Post-positivism is basically an abstract theory, based on the philosophy of knowledge. Most of its implications are based on the inability to assert, with certainty, that one's conclusions are true. Thus, all research results are potentially prone to error and can never be proven in any final sense. 

As such, it has very little to do with any aspect of research design, including both data collection and analysis. 

David L Morgan answered this so well. I however, find it challenging to distinguish Post-positivism from critical realism. I am finding it hard at the moment, to decide in which of the two philosophies should I locate my study considering that it is a non experimental quantitative research. Any advice?

If I were to define Post positivism, then it will a fisherman diving into the river to enquire about the fist rather than standing in the shore and observing like a positivist.

Post positivism can mean many different things and critical realism is very different from postmodernism which is itself differentiated.

I have written this piece on the practice of quantitative methods from a critical realist perspective Chapter The practice of quantitative methods

"A critical realist believes that there is an independent reality that science can study–each of us is not making it all up! It aims to develop deeper levels of explanation as causal necessity, not just regularity [a distinguishing feature of positivism] . It provides a logic of enquiry based on the fundamental formula:

Mechanism + Context = Outcomes

Mechanisms are not regularities but are potentially causal generative processes that operate in particular historical, local, or institutional contexts to produce particular patterns of outcomes."

You may also be interested in the answers to this question : https://www.researchgate.net/post/I-would-like-to-know-whether-critical-realism-is-compatible-with-purely-quantitative-research-method

I agree with David L Morgan as well. Fallibilism is an important epistemological stance here (which you also find in American pragmatism, like the philosophical work of Charles Sanders Peirce). Also think about Karl Popper's criticism of logical positivism and the falsification principle he proposed to be used instead of the verification principle advocated by logical positivism. If you would like to read more about post positivism as a philosophical basis for doing social research (in this case, educational research), I can recommend a very helpful book (but remember that the term "post positivism" can be interpreted and used in different ways, as Kelvyn Jones above mentions): https://psycnet.apa.org/record/2000-08253-000 . Good luck with your work!

It is progressive then positivism philosophy so has it single reality or multiple reality.I am little bit confused.

Excellent answer by Kelvyn Jones - read his chapter!