Unable to Retrieve Token Getting JSONDecodeError: Expecting value: line 1 column 1 (char 0)

I am trying to generate a token from a REST API server with the following Python code:

import requests
import json

username = "username"
# put your password inside, exemple : password = "mypassword"
password = "password"
payload = {
 'grant_type': 'password',
 'username': username,
 'password': password
}

r = requests.post("">sandbox.serversandbox.com/token", 
 headers={
 "Content-type":"application/x-www-form-urlencoded",
 "cache-control":"no-cache"
 },
 data=payload)
# this print command will return only the token
print(r.json()['access_token'])
# this print command will return the whole response
#print(r.text)
# this print command will return the status code (200 if ok)
# print(r.status_code)

But I'm getting the following error:

JSONDecodeError: Expecting value: line 1 column 1 (char 0)

And the error points to the following:

---> 17 print(r.json()['access_token'])

Can someone let me know what could be the issue?