Thank you for contacting us.
Could you please verify the following on your side:
- The server key ("API key") you created in Google Developer Console is entered in your backend project under Settings > Push notifications > Android.
- The Google API project number (12-digit number) is the value assigned to the GOOGLE_PROJECT_NUMBER variable.
- The Google Cloud Messaging for Android service's status is enabled for your Google project.
- The project is built and the deployed to a device through a USB cable connection.
The steps for setting up your Google project for push notifications are more extensively outlined in this article
of the official Android documentation.
Hopefully, this will help. Should any further questions occur, please, do not hesitate to contact us.