A customer says my coupon code doesn't work. How do I find out why?

The error the customer sees tells you which rule stopped it. Check the coupon in Marketing → Coupons:

• "Invalid or inactive coupon code": wrong code, or Active isn't ticked.
• "This coupon is not active yet" or "This coupon has expired": check Valid from and Valid until.
• "Order must be at least…" or "valid only on orders up to…": Minimum cart value or Maximum cart value.
• "This coupon has reached its usage limit" or "You have already used this coupon": Total usage limit or Per-customer limit.
• "valid on your first order only": First order only is ticked.
• "not available for your account": Customer eligibility.
• "No eligible items in your cart": Product eligibility.

The Used column on the list shows how often it has been redeemed.