online debit card payment declined what to do ??

Know it here --> If your payment method is declined in the App Store or iTunes ...

See the pic below

Also, check the following

1. The payment method that you are adding is of the same country as that of App Store Country on your iPhone
2. You have a valid payment plan debit/credit card from an authorised bank from the same country
3. Payment methods that you can use with your Apple ID - Apple Support
4. Learn more about family purchases and payments.
5. Please check if your Debit/Credit card is valid internationally

Try to add funds to Apple ID Balance: Based on your country of residence and country you have selected under your Apple ID --> Payment methods that you can use with your Apple ID - Apple ...

You may --> Add money to your Apple ID balance