If you have vintage iOS hardware, you may not be able to run the latest release of some apps (i.e. requires iOS 8). Some users have indicated success in purchasing, then downloading an earlier version of an app. Note that Apple indicates "Previously purchased items MIGHT not be available if they're no longer on the iTunes Store."
- Purchase the app using iTunes on your computer.
- After the app has completed download, go to your iOS device and follow the steps to Download past purchases.
- When you attempt to download a current app that requires a newer version of iOS, you should see an alert indicating "This is version X for iPad 8.1. Would you like an earlier version of this app?". Click "Yes".