PayPal Refund Transaction Authorisation Issue

I'm working on Odoo version 12.0, developing the process of Refund Sale Transaction using their API. I've taken reference from the following link: https://developer.paypal.com/docs/api/payments/v1/#sale_refund

But there is some authorization issue with their API of the refund transaction.

I've example request/response parameters like the following:

import requests

headers = {'Content-Type': 'application/json','Authorization': 'Bearer Access-Token'}

data = '{\n  "amount": {\n    "total": "2.34",\n    "currency": "USD"\n  },\n  "invoice_number": "INV-1234567"\n}'

response = requests.post('https://api.sandbox.paypal.com/v1/payments/sale/2MU78835H4515710F/refund', headers=headers, data=data)

It gives me the following response:

{'message': 'Authentication failed due to invalid authentication credentials or a missing Authorization header.', 'links': [{'rel': 'information_link', 'href': 'https://developer.paypal.com/docs/api/overview/#error'}], 'name': 'AUTHENTICATION_FAILURE'}

Can anyone help me to resolve this issue?

And how to generate bearer token using PayPal Payment API?