Why Does GET /customers Return No Data?

If you have created customer and order data in your WooCommerce account and yet an API call to GET /customers returns an empty array then your problem may be with guest orders. When Guest Checkout is enabled and a guest order is created the customer_id will be recorded with a value of '0', which causes issues retrieving the customer data via API call.

In the WooCommerce documentation they actually call this out and recommend that you do NOT enable guest checkout for API integration business cases since customers must have an account for the WooCommerce API Manager to securely handle API requests. So in order to create valid WooCommerce data that can be retrieved through GET /customers do not create orders with guest checkout or with the Administrator role, and instead create orders using either the Customer or Subscriber roles.

https://docs.woocommerce.com/document/woocommerce-api-manager/