A coupon with this code already exists
Admin coupon create/update rejected because the code is already used (admin-checkout-coupons).
Why this happens
Coupon codes must be unique. Editing another coupon to the same code, or creating a duplicate, triggers this. Not a member checkout error.
Diagnose and fix
1. Pick a different code, or edit the existing coupon instead of creating a new one.
2. Search the coupons list for the conflicting code (case/normalization may apply).
3. Deactivate/delete the old coupon if it should be replaced.
4. Members redeeming codes see different “invalid coupon” style messages.