Hi Rachana,
Reading this, you are executing a call REST activity in the PushNotifications.SendFCMMessage microflow. The server you are calling returns a 403 Forbidden response.
Generally this means you don't have access to the requested resource, although you are authenticated (otherwise I would expect a 401 Unauthorized).
Can you verify if you are using the correct credentials and that account has the appropriate level of authorization? Otherwise: tell a bit more about the service you are calling.
Good luck!