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.
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 users with the "Administrate" right can see the REST API key. You will need to ask the administrator of the account to share that information with you.
The administrator of the account can also grant you "Administrate" access from the company settings.