This module ignores stock levels if you (maybe) have Commerce Stock installed. This means that you can still add items to the cart through the webform, even though the item is unavailable. This results in the cart telling the user they can only add quantity 0 of the items, even though commerce webform order has added a quantity to the cart. At this point it's hard to tell that the issue is the item is out of stock and the resolution is to remove it from the cart.
Install Commerce Stock, set your item stock level to zero, then use the webform to add it to the cart. Note that Commerce Stock may not actually be required based on using the Commerce Core "Availalbility Manager" mentioned below.
Use the "Availability Manager" included in Commerce Core to determine whether the item(s) on the webform are available for purchase and react accordingly.
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