Please see this article if you are trying to create or edit a coupon.
Payments of less than $1
Stripe will say a coupon is invalid if it reduces the prices to between $0.01 and ~$1.00. Make sure the coupon is 100% off, or the remaining price is more than $1.00.
(SOLVED!) One-time Payments of $0
It is now possible to use 100% off coupons with one-time plans 🎉