Inconvenience caused by Truworths online shopping

I am so unhappy with Truworths online service. I ordered a pair of shoes and paid for them and this morning I receive a message that the order is not available and the order is canceled. What type of service is this? Why did the system allow me to purchase the item in the first place. I'm extremely unhappy with Truworths. They just don't understand the inconvenience and impact to their customers.