Feature 'Offer free gifts' is only supported with products that have selectable variants (options).
If your products have only 1 variant, please follow these steps to create more variants (options) to make this feature works.
Step 1: Go to your Weebly Admin Dashboard >> Store >> Products
Step 2: Choose the product you want to set up discount >> click Product Options
Step 3: Add product options for that product. You can freely set the Option Title and Option Choices as you want:
Step 4: Hit Save. Then Publish your site again.
And that's it. You're done!