In order to use Firebase notification, you must authenticate your requests to Firebase with Service Account credentials.
you need to generate a private key file in JSON format and add it to the panel directly. To generate a private key file for your service account:
1. Open https://console.firebase.google.com/project/_/settings/serviceaccounts/adminsdk and select the project you want to generate a private key file for.
2. Click Generate New Private Key, then confirm by clicking Generate Key
3. one Generated you can download the Json file.
4. now open the admin panel and go to Settings page and open Firebase Settings tab
5. open the Json file and copy the file contains and paste it in the Firebase credentials field and save.