I think you guys might have an additional version of the module somewhere. I downloaded the latest version of the Improved Coupon Discount module and there is no mention of that function anywhere - nor is there one in the original Coupon module.

Delete any folders with the name coupon and then re-upload the latest module (Improved Coupon).

In 1.5 the only functions with the term "pane" are uc_checkout_pane_coupon(). and .uc_coupon_checkout_pane().