1. Sign in to your Google Play Console and choose Setting in the left side panel.
2. Under Settings select API access. If you don't have a project linked, click Create new project.
3. After you have a project, scroll down to the Service Account section and click Create Service Account then click the Google API Console link in the pop up box.
4. Create a new service account.
5. Enter your Service account name and select a Role Viewer. Click Furnish a new private key and choose JSON. When you have finished, click the Save button and you will get a JSON file downloaded to your local machine.
6. Return to the API access in your Google Play Console and click Done.
7. Click Grant Access to the Service Account you just created. Granting access allows Facebook to verify in-app purchases.
8. Select Never for the Access expiry date and View financial data, then click Add User.