This may mean that the product is now out of stock. The product may have been available when you added it to your basket but by the time you have completed the checkout process the product is now no longer available. The site will 'save' this item in your basket, but will not reserve the stock. Unfortunately, this is more common during sale periods and product launches when products are in high demand.
Certain products are also unavailable in specific countries due to shipping restrictions (particularly fragrance & setting spray) however you should see a message alert in your basket if we cannot ship a product to your delivery address. You can check which countries this applies to HERE.