Android - Facebook Analytics - Push campaign setup verification not working

I implemented FB Push Campaigns in our Android app. However I seem to be not able to run the "Push campaign setup verification".

I have a few questions that I am unable to find an answer for:

1. Where does the "YOUR DEVICE TOKEN" come from? What device are we talking about in this context.
2. The status is saying:

The sender account used to send a message couldn't be authenticated. This is probably due to an invalid project number being sent as the key, or the key could be valid but with the GCM service disabled, or our servers have not been whitelisted in the server key IPs.

According to this article

the problem is that "GCM sender ID is invalid". I verified our GCM and it is correct. I am quite confused about what is actually wrong or how to properly work out the issue.

You need to provide GCM server key in Facebook settings for push notifications

Starting from Sept. 2016 new server key can only be created in the Firebase Console using the Cloud Messaging tab of the Settings panel. Existing projects that need to create a new server key can be imported in the Firebase console without affecting their existing configuration.

Device token is the registration token of your device which is generated when you first install your app after FCM/ GCM integration.