Why did I get API error code 11601?

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

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

Solution
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.