Here are some verification's to make that may resolve your purchasing issue:
- Make sure your WiFi is stable
- Reboot your device to reset all networks
- Verity that there aren't any updates to make on either the device or the app
- Make sure you are freshly signed into the device's account ID
- Verify that all payment information in your account are up to date
- Make sure you have not activated any restrictions concerning new app downloads in your device's settings menu
- Make sure your device's Time & Date are set to automatic.