Coupons are not available
Applying a coupon threw Coupons are not available. (checkout-coupon when the feature/path is disabled).
Why this happens
Site coupon checkout is turned off, or this product surface does not accept coupons. Distinct from a bad code (Coupon code is not valid).
Diagnose and fix
1. Admin: enable coupons / checkout-coupon settings if members should use promos.
2. Member: pay without a code, or use a surface that supports coupons.
3. Confirm the coupon itself is still active once the feature is on.
4. See related invalid/apply articles when the feature is enabled but the code fails.