If you are reviewing carts you may see an item in a customers cart that you know is out of stock. No worries. When the customer goes to checkout, they'll receive a warning message: "<Product> has been removed from your cart because it is no longer available". This situation can occur when they add a product that is currently in stock to their cart and before they checkout the item sells out. This is a rare scenario, but it can happen.
This warning is to avoid members from adding products to their carts, then leaving and coming back and trying to buy it, when you no longer have inventory.
If you get this error while trying to checkout, it's because there is no inventory left for the product you are trying to add. If you are logged in as admin, it is likely because from a previous session, you had something in your cart that is now no longer available.
If you do have inventory, you should review and update your inventory.