The Firebase Cloud Messaging (FCM) API you're using is outdated or deprecated. Google has transitioned from using the Legacy HTTP Protocol to HTTP v1 API for sending FCM messages, and the API you're using (fcm-node) likely still uses the old protocol.
Existing send method gives below error:
{"multicast_id":8968610879307183290,"success":0,"failure":1,"canonical_ids":0,"results":[{"error":"DeprecatedApi"}]}