This problem was reported over the weekend and SP are dealing with it. You can make a payment by phone if the App is not working, 0800 027 0072.
I don't think you are on PowerUp which is a completely different service to Pay As You Go.
Hi @cathyjoy, sorry to hear you're having issues topping up. The smart prepayment fuctionality isn't available in the PowerUp app, it's currently only available in the ScottishPower Your Energy app. Can I ask you to try downloading the Your Energy app and check if you can top up then?