TypeError: 'NoneType' object is not subscriptable on GAE instance assigning JSON to python variable

I’m getting an error TypeError: 'NoneType' object is not subscriptable when assigning JSON to a python variable like this sheet_id = data["sheetID"].
It only happens on my Google App Engine instance. I don’t get it when running my Flask app locally and sending POST requests to the app with Postman. Here’s the code snippet

@app.route('/main',methods=["POST"])
def main():
    data = request.json
    print(data)
    scope = [
    'https://www.googleapis.com/auth/spreadsheets',
    'https://www.googleapis.com/auth/gmail.send',
    'https://www.googleapis.com/auth/gmail.settings.basic',
    'https://www.googleapis.com/auth/gmail.settings.sharing'
    ]

    GCP_PROJECT = os.environ['GCP_PROJECT']
    GAE_SID_VERSION = os.environ['GAE_SID_VERSION']


    credentials1 = access_secret_version(GCP_PROJECT, <SECRET NAME>, GAE_SID_VERSION)
    credentials1 = json.loads(credentials1)
    credentials = service_account.Credentials.from_service_account_info(credentials1)
    scoped_credentials = credentials.with_scopes(scope)
    gc = gspread.authorize(scoped_credentials)

    #for sheet with categories and score
    sheet_id = data["sheetID"] #this gives me the error

    workbook = gc.open_by_key(sheet_id)
    ...

    ...
    return "OK"

The strange thing is that the whole program runs to completion and I receive response = OK. I’m able to send json data from a triggered script in Google Sheets to my GAE instance, then the GAE instance loads data into a Gcloud storage bucket later. I understand from this link that either data or sheet_id could be None but then in that case how would my full program run to completion or even just open the workbook? Here’s the Google Apps script

function runDockerImage(sheetID , mainWorksheet , emailTo , emailList , deliverToCloud ,filterOn , labelDisplay , sandbox , deliverToSlack) {
  // The code below logs the HTML code of the Google home page.
  var dataJSON = {
    'sheetID' :sheetID,
    'mainWorksheet' : mainWorksheet,
    'emailTo' : emailTo,
    'emailList' : emailList,
    'deliverToCloud' : deliverToCloud,
    'filterOn' : filterOn,
    'labelDisplay': labelDisplay,
    'sandbox' : sandbox,
    'deliverToSlack' : deliverToSlack 
    
  };
  
  payload = JSON.stringify(dataJSON)
  
  var headers = {
    'Accept': 'application/json',
    'Content-Type': 'application/json',
    'Authorization': 'Basic _authcode_'
  };
  
  var options = {
     'method': 'post',
     'contentType': 'application/json',
     'headers': headers,
     'payload': payload
  };
  
  
  var response = UrlFetchApp.fetch('https://<project-id>.uc.r.appspot.com/main', options);
  Logger.log(response);
 
}

Here’s the tracback from App Engine Error Reporting log:

Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python3.6/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/app/app.py", line 118, in main
    sheet_id = data["sheetID"]
TypeError: 'NoneType' object is not subscriptable

Finally, I’d just like to ask how is it possible that this program runs completely with this error? I think that I might run into some serious problems later if I don’t fix this.

Screenshot of the Error Reporting Log