To use Push Notifications in Google Calendar make sure you followed the instructions here:
- Register the domain of your receiving URL in Google Dev Console. Go to Credentials and click Domain Verification for the webhook setup.

For example, if you plan to use https://yourdomainexample.com/notifications
as your receiving URL, you need to register https://yourdomainexample.com
.
Set up your receiving URL, or "Webhook" callback receiver.
This is an HTTPS server that handles the API notification messages that are triggered when a resource changes.
Set up a notification channel for each resource endpoint you want to watch.
A channel specifies routing information for notification messages. As part of the channel setup, you identify the specific URL where you want to receive notifications. Whenever a channel's resource changes, the Google Calendar API sends a notification message as a POST request to that URL.
You may also check this SO thread for additional reference.