Why did I get API error code 11601?

PayPal returned API error code 11601: Request for billing address failed

Your request for the customer's billling address failed and the SetCustomerBillingAgreement isn't enabled for your merchant account.

Requesting customer's billing address is available only when vetted and approved for use of this feature. Contact PayPal's Customer Service to discuss enabling the Request Billing Address option on your PayPal account.