Product Discount works by changing the product price. The app does not have the ability to change shipping rates.
Product Discount works the way other online and brick-and-mortar stores run sales, by reducing the product price when products go on sale, and return to regular price when the sale ends. You can schedule the sale to start and end at any day/time you need. Discounts are applied as either a percentage off, or a price discount from the regular price.
To offer free shipping, suggested alternatives are creating a Shopify discount coupon code that customers can use in the store checkout, or change your shipping rates.
You can navigate here by going to Shopify Admin > Discount - 'Add Discount', here you can create free shipping for any shipping rate that is less or equal to $XX.XX and applies to 'All Counties', 'Canada' or the 'Rest of the World'.