If you have one of the new iPad's without a Home Button, like the iPad Pro, then you may have encountered the situation where the iPad asks to to "Double Click to Pay". This is quite difficult as you need to quickly press the side button of your iPad with a lot of pressure. If your iPad is in a protective cover, it is even more difficult.
- Open the iPad Settings app, and scroll down to Face ID & Passcode
- In the section "Use Face ID For" turn it off iTunes & App Store
- Open the iPad Settings app, and go to the Accessibility
- Tap the blue icon for "Top Button" (could also be called "Side Buton"
- Turn on the option to "Use Passcode for Payments" instead of double clicking the button.