Yes, you do have to receive the item back first before refunding.
Message the buyer of this missing item, and if they wish to return the item for a full refund, they need to open a case for item not as described.
Accept the case within 3 days and provide that paid for tracked return label. Only on receipt of your item, do you fully refund, through those case details. You'll have 48 hours to do so.
They may be happy with a partial refund, but this is solely between you and the buyer to both agree on this.