GoCardless Incorrect token error

gistfile1.py

import gocardless
from api import settings

# set environment to sandbox
gocardless.environment = settings.GOCARDLESS_ENVIRONMENT

gocardless.set_details(
    app_id=settings.GOCARDLESS_MERCHANT_APP_ID,
    app_secret=settings.GOCARDLESS_MERCHANT_APP_SECRET,
    access_token=settings.GOCARDLESS_MERCHANT_ACCESS_TOKEN,
    merchant_id=settings.GOCARDLESS_MERCHANT_ID
    )

def get_gc_partner_authorization_url(merchant_details):
	"""
	passes merchant details to GoCardless and returns a URL, where a merchant can authorise with GoCardless
	info: https://developer.gocardless.com/python/#merchant-account-authorization
	"""

	client = gocardless.Client(
	  settings.GOCARDLESS_PARTNER_APP_ID,
	  settings.GOCARDLESS_PARTNER_APP_SECRET
	)

	url = client.new_merchant_url(
	  redirect_uri='http://httpbin.org/get',
	  merchant=merchant_details
	)

	return url