GCM Error=MissingRegistration sending messages via JSON

For Firebase, You can get info from https://firebase.google.com/docs/cloud-messaging/http-server-ref#error-codes

Check that the request contains a registration token (in the registration_id in a plain text message, or in the to or registration_ids field in JSON).

For the new cloud Message, when you want to send a dwonstream message from the server, you need to use "to" to declaim the target registration id.

like below :

https://gcm-http.googleapis.com/gcm/send
Content-Type:application/json
Authorization:key=AIzaSyZ-1u...0GBYzPu7Udno5aA

{ "data": {
    "score": "5x1",
    "time": "15:10"
  },
  "to" : "APA91bHun4MxP5egoKMwt2KZFBaFUH-1RYqx..."
}

You probably forgot to specify the content type in the header to be JSON.

Content-Type: application/json

If Content-Type is omitted, the format is assumed to be plain text.

And for plain text the registration ID is passed in a parameter called registration_id instead of registration_ids, which explains your MissingRegistration error.