The reason might be due to the setting of your device, the store application and available payment options and balance.

Please try the following:
1) Check the Internet connection.
2) Check your selected payment method and available balance.
3) Check the system date on your device to make sure it corresponds to the real date and time. To avoid this problem, enable automatic syncing in the date and time settings.
4) Check your device for any hacking applications. We verify purchases and such applications invalidate purchases.
5) Reinstall the application.
6) In case of Google Play — clear the cache of the Play Market application. Go to Settings -> Applications -> Google Play Market -> Memory.