How to use sandbox accounts in iOS 11?
One possibility is that you have different versions of iOS. As per this documentation:


For iOS 11 or earlier — Sign out of the App Store in Settings, then build and run your app from Xcode.
For iOS 12 or later — Don't sign out of the App Store; simply build and run your app from Xcode. Sandbox accounts are stored separately, and you can control your sandbox account directly on-device in Settings.


Additional related documentation: 


Test in-app purchases - App Store Connect Help
Apple Pay - Sandbox Testing - Apple Developer