The free gift isn't added to the cart

If you configured a free gift on an offer but it never appears in the cart, the most common cause is that the gift product isn't published to the Online Store sales channel. Shopify blocks any product from being added to the cart when it isn't available on that channel, even as a free gift.

How to fix it

Open the gift product in your Shopify admin. In the Publishing (or Sales channels) card on the right side, check whether Online Store is listed. If it isn't, click Manage and enable it, then save.

You don't have to show the product anywhere on your storefront. Publishing to the channel only makes it addable to the cart. You can keep it out of your menus, collections, and search so customers never browse to it directly.

Other things to check

The gift must be in stock, or have "Continue selling when out of stock" enabled, otherwise Shopify refuses to add it.

A product cannot be its own free gift. Shopify rejects a discount where the qualifying product and the gift are the same item. Use a Buy X Get Y offer for that case instead, see how to set up a Buy X Get Y offer.

Free gifts are added through the regular add to cart flow. If your customers use the express Buy it now button, the gift can't ride along, see why the Buy it now button disappears on a bundle.