The "Oops--go to the store to scan this product" or "Location isn't accurate enough" error message will pop up when you're not in the store yet and trying to scan, or when your GPS hasn't updated since you got to the store.
If you are sure you're in the right store (the address listed in the app matches the one you're shopping at), refresh your GPS. You can do this by pulling down on the store list. This will update your app's location in case we have an outdated reading. You may need to go outside the store to get a strong signal, but once you do, you should be able to scan in that location.
Another reason you might run into this issue would be if your phone has location-faking software installed, mock locations enabled, or has been jailbroken. Any of these may affect your ability to scan items successfully, and will likely result in an error message. Just make sure none of the above applies to you, and you should be able to scan just fine.