Hi Llanut,
This text is not in the translation, we will correct it in the next release.
If you wan't to edit it by yourself, edit the file: yourWebsite/administrator/component/com_hikashop/classes/discount.php
And edit the line:
JRequest::setVar('coupon_error_message','Coupon has no value when used with current product discount(s).');
To:
$couponErrorMessage = JText::_('NO_VALUE_COUPON_WHEN_DISCOUNT');
JRequest::setVar('coupon_error_message',$couponErrorMessage);
And add:
NO_VALUE_COUPON_WHEN_DISCOUNT="Coupon has no value when used with current product discount(s)."
In the translation file.