Scan launches the ZXing Barcode Scanner, which you must have installed to scan items. You don't need to press the shutter button—just aim your device's camera at the item's barcode.

If the barcode scanner can't find your item, consider changing your locale in Settings. If you're scanning items from a retail store, make sure to scan the actual barcode of the item, not the barcode provided by the retailer—this is sometimes a sticker that's colored differently, or even pasted over the real barcode.