Do you fulfill the requisites from the documentation ?
Note: The Customer Information API section only returns customer information for Webstores with Seller-Branded Checkout. No customer information is returned for Webstores with Amazon.com checkout. If your Webstore offers both checkout options, customer information is returned only for Seller-Branded Checkout accounts. For more information about Seller-Branded Checkout, see “Your Amazon Webstore Checkout Options” in the Amazon Webstore Help: (US) (UK).
Important: In addition to registering for Amazon MWS, you must request authorization to use the Customer Information API section. See “Webstore API Management” (US) (UK) in Seller Central to request authorization for the Customer Information API section, either to develop your own applications or to enable third-party developers to develop applications for you. The Customer Information API section is available only in the United States and the United Kingdom.