appian postman

I have created an apian web post API. api is working in appian and when i try to test it in postman its gives error 501. how can i test it in post man. need help. i already created outh credentials and also added the service user to the application groups.  

a!localVariables(
  local!value: cast(
    'type!{urn:com:appian:types:TFA}TFA_TESTING_RT',
    a!fromJson(http!request.body)
  ),
  a!writeToDataStoreEntity(
    dataStoreEntity: cons!TFA_data_store_constant,
    valueToStore: local!value,
    onSuccess: a!httpResponse(
      statusCode: 200,
      headers: {
        a!httpHeader(name: "Content-Type", value: "application/json")
      },
      body: a!toJson(
        fv!storedValues
      )
    ),
    onError: a!httpResponse(
      statusCode: 500,
      headers: {
        a!httpHeader(name: "Content-Type", value: "application/json")
      },
      body: a!toJson(
        {
          error: "There was an error writing to the data store"
        }
      )
    )
  )
)