Firebase FCM error: 'InvalidRegistration'

In my case, I was sending notifications to topic ("topics/my-topic"). I was missing prepending / in the starting of topic so I was getting the same issue. SO topic should be /topics/my-topic.

May be this helps!!

There is an easier way to send a message to a device group from a Cloud Function. Use admin.messaging().sendToDeviceGroup(). Sample code and instructions are in this guide.

I think your current method is failing because there is something wrong with the group notification key provided in groupId. It should be the string key value that was returned when you created the device group. The error codes are listed in this table. For 200/InvalidRegistration it says:

Check the format of the registration token you pass to the server. Make sure it matches the registration token the client app receives from registering with Firebase Notifications. Do not truncate or add additional characters.