Why can’t I add a product with the Quick Design Bar?

Check your Inventory Control Settings.

With the Quick Design bar, you are able to add products to your store on the fly. However, in some cases, after adding a product, the store may give you an error similar to the following:

The item could not be added to your order. It’s not currently available.

This particular error is caused when your inventory control settings are set to hide items when they are out of stock.

When you add a product via the admin design bar, the stock for the item will be set to 0 by default. (inventory can be edited through a different admin design bar link). After adding the product through the quick design bar, the store is tailored to display the product for you from the front-end store so you can edit it further. However, if the store is also set to hide the products with 0 stock, you will get the error mentioned above.

To correct this, simply set your Inventory Control to show items as “Out of Stock” when they reach 0 in inventory.
(Settings ->General -> Store Settings | General Tab)