To have a Freebie—a free bundle—you must choose a new bundle option.
Navigate to Bundles and click on Add New (or Add New Bundle if this is your first bundle)
Select the Freebie option, then continue to create your bundle
NOTE: If you want the bundle to be available on your storefront right away, make sure you click on Available below the title. Once you add content to it, any customer viewing the checkout page for that content will see the Freebie bundle option.