must add the column to the db, as well as to the code.

docs/changelog/7.x.x.txt

@@ -16,6 +16,7 @@
  - fixed: Date picker shows time after moving through months
  - fixed: Cannot Instantiate Template
  - fixed: Adding Donation Asset breaks style template
+ - fixed: Adding Flat Discount Coupon causes "Problem w/ Request" error
 
 7.5.12
  - skipping this release

docs/upgrades/upgrade_7.5.12-7.5.13.pl

@@ -30,7 +30,7 @@ my $session = start(); # this line required
 
 fixShop($session);
 addSelectableProfileTemplates($session); 
-
+addCouponThankYouMessage($session);
 finish($session); # this line required
 
 
@@ -57,6 +57,13 @@ sub addSelectableProfileTemplates {
     $session->setting->add('editUserProfileTemplate', $tmpl);
 }
 
+#----------------------------------------------------------------------------
+sub addCouponThankYouMessage {
+    my $session = shift;
+    print "\Adding Thank You Message to Coupon table... " unless $quiet;
+    $session->db->write('alter table FlatDiscount add column thankYouMessage mediumtext');
+}
+
 
 # -------------- DO NOT EDIT BELOW THIS LINE --------------------------------