Unfortunately this can happen sometimes. If you are unable to add an available product to the cart, it may be that the item or size you are interested in, has sold out. The website updates its stock levels almost instantly but it's not in real-time.
This generally only happens with popular items which sell out quickly. These items generate a huge amount of traffic to the website within a short time-frame, not giving the system adequate time to refresh.
We advise that you try to purchase the item again later, or visit one of our stores!