Error Using Payflow with Magento Enterprise Edition (EE) 1.12.0.2

Problem

A customer applies a coupon code when checking out in a web store that runs EE 1.12.0.2 and is configured to use the Payflow Pro or Payflow Express payment methods. The following error displays:

Solution

To resolve this issue, contact Magento Support and request the patch for support issue ID SUPEE-1474.

Then apply the patch as discussed in How to Apply and Revert Magento Patches.