Service Key is only available for Cloud Messaging API (Legacy) and by default, Firebase Cloud Messaging API (V1) is the only one enabled.

To enable Cloud Messaging API (Legacy), please follow the steps below: 

    1. On your Firebase account, select the project and navigate to Project Settings.

    2. Go to the Cloud Messaging tab and click on the 3 vertical dots on Cloud Messaging API (Legacy) and go to "Manage API in Google Cloud Console" 

    3. It should redirect you to the page to enable Cloud Messaging. Simply click "Enable" and the API will be loaded.

    4. Go back to the Google FireBase page and reload. Cloud Messaging API (Legacy) should be enabled now and Service Key will appear.