This coupon has expired or is disabled
Coupon validate/apply returned premium.couponExpired — the code exists but is not currently usable.
Why this happens
validUntil passed, enabled false, or outside the valid-from window. Distinct from You have already used this coupon (per-user one-time) and unknown codes.
Diagnose and fix
1. Member: use another code or continue without a discount.
2. Admin: extend dates and/or re-enable the coupon; save.
3. Confirm site clock/timezone is not far off if coupons expire unexpectedly.
4. See Enter a coupon code / Coupon code is not valid.