Your selected item might have a limited stock quantity. If you are unable to select the item to your cart, there are high chances that someone else already did and the product is now out of stock. Kindly refresh the page after¬ for the stock to be updated and you may try again.
When you added your desired item to the cart, it does not guarantee your booking of the item. If you do not proceed to checkout and made payment immediately, your item still is subjected to change in stock, price and discounts rule.