This message typically occurs when your website's redirect to the ticket form is not the accurate URL. Verify you can add the discount to your cart using the correct URL with the steps below:
  1. Navigate to Web> Manage program forms
  2. Click drop down next to program event
  3. Click hyperlink to URL
  4. Test adding tickets to cart and adding discount
  5. If issue no longer persists, you need to update the URL in your website's redirect to this correct URL

If this issue persists, please refer to Ticket discount is not applying to a sales order (includes video demo)