The REST API key is sensitive information as it allows you to use our API to send push notifications, create custom audiences, attach data to specific users, and more (see more here).
You will find it in the dashboard settings, in the "General" tab. You may see "insufficient privileges" in the REST API key field.
This happens because only managers can see the REST API key. If it's the case, you will need to ask the manager of the account to share that information with you. The manager of the account can also grant you "manager" access from https://login.batch.com/team.