Inserting and updating Custom Dimensions using Google Analytics Management API v3 - python-3.x

Question

I am using Google Analytics API to insert a custom dimension. I have set up the required Oauth2 credentials and obtained an access token and a refresh token. However, when I called my function that inserts a custom dimension to a specified Account and webProperty in Google Analytics, I get an error message that reads as follows:
<HttpError 403 "Request had insufficient authentication scopes.". Details: "[{'message': 'Insufficient Permission', 'domain': 'global', 'reason': 'insufficientPermissions'}]">
However, using the same code, I was able to fetch all accounts and webProperties in Google Analytics but apparently it can neither insert nor update custom dimensions and I don't know what I am missing in my code.
Here is my code:
from apiclient.discovery import build
from oauth2client.service_account import ServiceAccountCredentials
from googleapiclient.errors import HttpError
import os
from google.oauth2.credentials import Credentials
from google.auth.transport.requests import Request
scope = ['https://www.googleapis.com/auth/analytics']
def get_service():
"""
authorises user to access GA resources and builds a service object
"""
creds = None
# The file token.json stores the user's access and refresh tokens, and is
# created automatically when the authorization flow completes for the first
# time.
# It can also be created manually and populated with the required credentials
if os.path.exists('token.json'):
creds = Credentials.from_authorized_user_file('token.json', scopes=scope)
# If there are no (valid) credentials available, let the user log in.
if not creds or not creds.valid:
if creds and creds.expired and creds.refresh_token:
creds.refresh(Request())
else:
flow = InstalledAppFlow.from_client_secrets_file(
'client_secrets.json', scopes=scope)
creds = flow.run_local_server(port=80)
# Save the credentials for the next run
with open('token.json', 'w') as token:
token.write(creds.to_json())
# build a service object
service = build('analytics', 'v3', credentials=creds)
return service
def insert_customDimension(service):
# Inserts a custom Dimension at a specific index.
try:
service.management().customDimensions().insert(
accountId=ACCOUNT_ID,
webPropertyId=WEB_PROPERTY_ID,
body={'name': 'free to use',
'scope': 'HIT',
'active': False,
'index': 200
}
).execute()
except HttpError as error:
print(error)
except TypeError as er:
print(er)
I would like to understand what I am doing wrong or what do I need to change in my code?
I have deleted the token.json file and ran the code but I was unable to complete the authorization flow and no token.json was automatically created. I then manually obtained the refresh and access token and created the file. I have admin privileges to the Google Analytics account I am trying to insert custom dimensions into.