Paid but not received 3 months already

I put in an order (SSO2536) in October 2020 and paid for it at the same time. When I received the order, There was one product short. I was not informed when I ordered that it was out of stock, just told by the person who delivered. I called on 23 October 2020 and sent NUMEROUS emails afterwards but my requests of a refund or delivery of the product was simply ignored. Surely a large business like this can look after their customers and have the common decency to reply and rectify the matter.