Everything looks to be fine, I have my filtered list with a sensable audience of 4. Everytime someone downloads the Android APP is added to the OptsIN Contacts. But notifications still not sendind. In the records it only states X Error FOR Status, without
When the device registered but you don't receive push notifications means its mostly due to incorrect Application Provisioning.
When you provision your app using Firebase(FCM), you will get the Server key, Legacy Server key and Senderid.
The Legacy Server key is the API key to be used for the GCM Client. Make sure you have used the Legacy Server Key.