There could be the following reasons why you can’t make purchases (you've been able to make purchases before):
1.Your Internet connection is not stable. We recommend using Wi-Fi or at least a 3G/4G connection.
2.Temporary problems with the application store. Sometimes the Store is overloaded and unable to proceed your request. Please, wait for a couple of minutes and try again.
3.Problems with the store app. You may solve this issue by restarting your device.
There could be the following reasons why you can’t make purchases (you've never been able to make purchases):
1.Incorrect payment information. The simplest way to fix that is to cancel the payment method and start over.
2.Unsupported payment method. It may be that the payment method you use is not supported by the application store. The list of available payment methods can be found in the respective sections of the Apple and Google customer support websites.
Contact the application store customer support if you’re still having troubles making purchases.