This article has been ported to Troubleshooting Paypal Website Payments Pro errors

Gateway Response: (10501)  

According to the paypal documentation, this error 10501 relates to a billing agreement in paypal account settings

Paypal account should be checked to ensure if has been correctly setup.

Gateway Response: (10561)

When using PayPal Website Payments Pro to collect payments, the billing address system fields are a mandatory option.

You will need to add Billing details field , from your Form Builder to the Online Purchase Form

PayPal Website Pro -- Gateway Response: (10002)

This message is being given to the system by the gateway, and typically occurs when the credentials being entered into the system are incorrect.    The error #10002, means two things according to PayPal:

a) Authentication/Authorization Failed:: Username/Password is incorrect    This error can be caused by an incorrect API username, an incorrect API password, or an invalid API signature. Make sure that all three of these values are correct. For your security, PayPal does not report exactly which of these three values might be in error.

or

b) Authentication/Authorization Failed :: You do not have permission to make this API call

We suggest you verify if the gateway is properly set – and if so, PayPal should be contacted to confirm why this error is being generated.

Gateway Response: (15005)

According to PayPal Documentation

15005: This transaction cannot be processed. Please enter a valid credit card number and type. - The transaction was declined by the issuing bank, not PayPal. The merchant should attempt another card.

This error code is in fact returned by PayPal and not by Business Catalyst which indicates that the payments are successfully getting pushed to PayPal. I would suggest contacting PayPal in regards to this error code for further clarification.