Why did I get API error code 11601?
We return this NVP/SOAP API error code when the request to acquire the buyer’s billing address fails. This feature is only available to pre-identified merchants.
Requesting the customer's billing address is available to pre-vetted and approved merchants. Unless already pre-identified, there isn’t a method to request this feature to be enabled.
See the Developer Portal for a complete list of NVP/SOAP API error codes.