@Johnboy20002002 SP says
Once you’ve downloaded our free app, you’ll need to register a debit or credit card to your account. You’re then ready to top up via the app. Simply select the ‘Top Up’ icon on the home screen. You can choose between gas and electricity and decide how much you want to top up – the minimum amount is £1. Your credit can take up to an hour to appear on your meter. Find out more about topping up here.
Full instructions are here
@Johnboy20002002 I cannot check because I am not on PAYG, but the instructions say that you must select elecricity or gas on the App before you put in money. You don't need barcodes if the App is registered to your account, the App only needs the details of a credit or debit card. So you need to turn on the App and select gas before doing anything else.