1)Make sure with your payment provider if you have not turned off online payments for your credit or debit card. You will be unable to register your card as a payment method unless you've enabled online payments and, as a result, not be able to buy games off of the PS Store.
Make sure that the card details are up to date and accurate. To check your payment details, go to Account Management and then to Payment Management. Then, you have to select the payment card.
2)Try to avoid using any special characters in your name, address/flat numbers. If there are any, replace them with the nearest alphabet. Try not to use any and all asterisks, hyphens or slashes.
3)The importance of a stable internet connection cannot be stressed enough when it comes to this matter. It is very easy for the transactions to not go through if, at a crucial moment, the internet becomes unstable. Do not perform the transaction if your internet connection is unstable. Instead, wait until it gets better, or even better, switch your network to something else while performing the transaction.