Please see this article if you are trying to create or edit a coupon.
One-time Payments of $0 (or close to $0)
According to Stripe: “Checkout does not currently support discounting a purchase below the minimum charge amount for one-time payments.”
Update! - They are actively working on a fix and it might be ready by end of Q1 2023. If this is an issue for you, please reach out to Stripe support so it remains a priority for them.
So in other words, you can’t apply a 100% off coupon to a one-time payment in Checkout. We did speak to Stripe about this, and they have plans to add support for this at some point.
In the meantime, we recommend creating a free plan with the same gated content and redirects. You can manually give your member the free plan so they don't need to enter any payment data. If collecting payment information is important, you can create a recurring payment (every 1 year) with the same gated content and redirects. Stripe will allow the customer to enter their card and check out with a 100% off Coupon.