Why am I having problem to purchase ?

Have you tried going on the link it showed? There might be a problem on your payment details.

You can follow these steps:

- go to http://www.apple.com/support/itunes/ww

- click on your country's flag

- click on the Contact Support at the bottom of the left-hand column

- click on Contact iTunes Store Support on the right-hand side of the page

- then try Purchases, Billing & Redemption

Have you had a payment method declined in the App Store or iTunes Store https://support.apple.com/en-us/HT203005