Alertmanager notification failure to GChat(Google Chat) Webhook

Issue

• Alertmanager is failing to send notifications to GChat Webhook.
• In Alertmanager, a receiver with Google Chat Webhook endpoint specified has been created.
• Even when relevant alerts are fired, they are not notified to Google Chat Webhook.
• After the failure of notification, AlertmanagerClusterFailedToSendAlerts alert was fired. The details of the alert suggested that the cause was the endpoint is not reachable. However, the Webhook endpoint has been confirmed to be correct.
• How can we receive notifications from Alertmanager in GChat Webhook?