You have already used this coupon
Coupon validate/apply returned premium.couponAlreadyUsed — this account already redeemed a one-time (or limited) coupon.
Why this happens
Coupon is marked one-time per user and a prior successful apply/checkout used it. Distinct from expired/disabled and “does not apply to this plan”.
Diagnose and fix
1. Member: continue without the code, or use a different promo if you have one.
2. Admin: if a re-issue is intended, clear usage / create a new code (do not expect the same one-time code to work again).
3. Confirm the member is on the same account that already used it.
4. See This coupon has expired or is disabled if the code is also inactive.