This is because there are no credit card details that are captured if a customer uses a 100% discount coupon in a Webflow e-commerce store.
There needs to be a purchase at point of sale for at least $0.50 or else Webflow automatically hides the credit card portion of your checkout page.
If there are no credit card details stored... there is no way for us to create a subscription for them.
The way to get around this is an extended free trial on a subscription plan or simply refunding the invoice in your Stripe account for the subscription that you want to be free. (You can also set a very long free trial period on the individual Stripe subscription if you wish as well, after refunding the original payment.)