Hi,
Please edit the file "administrator/components/com_hikashop/classes/discount.php" and replace line 222
if (!empty($coupon->discount_product_id)) {
By
if (!empty($coupon->discount_product_id) && is_string($coupon->discount_product_id)) {
It will fix the warning and should improve the discount on the product.
But I see in your configuration that you have also a category selected but without a name ; please be sure that this category is removed from the discount configuration ; it might also affect the coupon processing.
Regards,