401 error for contact imports

I am trying to import contacts via api using python. But I am getting 401 error as :-
{"meta":{"httpStatus":"401 - Unauthorized","error":{"errorMessage":"No default directory found for brand: xyz.","errorCode":"xyz"},"requestId":"xyz"}}
My code is given below :-
import requests
import json

dict_Contacts = [
 {
  "firstName": "xyz",
  "lastName": "xyz",
  "email": "xyz@xyz.com",
  "language": "xyz",
  "unsubscribed":1, 
  "externalReference":"xyz",
  "embeddedData": {"ExampleField1":"xyz", "ExampleField2":"xyz"}
 }
]

Token = 'My_Token'
MailingListID = 'My_Mailing_List'
dataCenter = 'My_Data_Center'

url = "https://{0}.qualtrics.com/API/v3/mailinglists/{1}/contactimports/".format(dataCenter, MailingListID)
headers = {
  'x-api-token': Token,
  'contentType':'application/json'
  } 
json_string = json.dumps(dict_Contacts)
files = {'contacts': ('contacts', json_string, 'application/json')}
request = requests.post(url, headers=headers, json={'contacts':dict_Contacts})
print(request.text)

My token and path are correct. Can anybody please tell me what exactly the error means? (as No default directory found for brand) and how to resolve this issue.