I am having trouble in purchasing

Please check ⤵️

1.

See -> Add a payment method to your Apple ID -> Add a payment method to your Apple ID – Apple Support (UK)

• if you use your Apple Account balance, you need to set your Apple Account balance as your default payment.

If you have set your Apple Account balance as your default payment method, your default payment method will be used first by Apple when you need to make a payment.

Some purchases might require you to have a credit card on file, even if you use your Apple Account balance to make the purchase.

2.

See the following Apple articles:

Check your Apple ID balance -> Check your Apple ID balance – Apple Support (UK)

Add money to your Apple Account balance -> Add money to your Apple Account balance - Apple Support

3.

If your payment method is declined in the App Store or iTunes Store, see -> 

If your payment method is declined in the App Store or iTunes Store - Apple Support

Scroll to:

How to complete your purchase if a payment failed or was declined

1. On your iPhone or iPad, open the Settings app.
2. Tap your name.
3. Tap Payment & Shipping.
4. Add a different payment method and remove the old one. 
5. Try to complete your purchase.

If you still can't make a purchase

• You might have an unpaid order. Learn how to pay an unpaid order.
• Contact your financial institution. 
  • You might need to ask your financial institution to allow international purchases on your debit or credit card.
  • If you want to know why a debit card, credit card, or other payment method was declined, speak with your financial institution. Apple doesn't have information about why the payment method was declined.
• In some countries and regions, your payment method might be declined because of changes to local law. Learn about billing and purchases in Europe or what to do if your card is declined in India.
• Still need help? Contact Apple Support.

If you have followed all the steps and instructions and if you still need help, get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK) 

Hello ~ What sort of trouble? Technical details about your issue are needed for us to assist you. What happens when you try to ma a purchase ? Message? Post back with details.

~Katana-San~

Thank you for your feedback and you're welcome.