Unhappy with my online order not delivered

I placed an online order on the 6th of December 2023. Delivery time 2 - 5 days delivery. Today is the 18th of December 2023. Still no delivery. After numerous calls, today I was told - out of stock, and I have to wait for now 3 weeks. Why don't you have on your online orders out of stock notice. I need answers from Management.