The SmartStart in the mobile app is not very user friendly or intuitive.
You have to close that screen and go back in (or quit the app) to force it to refresh the info. It will always say pending right after you scan the code. There is a good chance the device is already included and you just did not realize it.
I suggest keeping the system logs open while doing SmartStart and you will see when the device pairs.
Someone has a similar issue recently, see this post and below: Smart Start on A GE Enbrighten Smart Outlet ZWA1003 - #34 by jtp10181 (might be some good info above also, sorry its a long thread)
You can also use this app on he hub to see details: [APP] SmartStart Manager for Z-Wave (Long Range support)