Discounts can be applied to the product variant only. Discounts can be applied to the product variant while the product variant is being created, or anytime thereafter. See https://www.nopcommerce.com/Documentation/SetUpYourCatalog.aspx for more info - Managing products section. Adding products differs from Editing products. When you click Add new button you start to add one product and one product variant. Later you can add another product variants. When you edit the product you start to edit only the product without product variants. To edit product variants of existing product go to Product Variants (SKUs) tab of the product.