Hi shasank50,
Welcome to the Apple Community, where some of us do our best to help each other troubleshoot.
(We are all users of Apple products here, just like you, so Apple will not read what you have written).
I understand you have a billing problem when you try to download apps from the App Store.
You might want to log-in to your Apple ID account, and see if everything is ok there.
Your Apple ID account is used for everything Apple, including App Store, iTunes, Music, iCloud, subscriptions etc.
It may be something simple, like updating the expiry date on your main payment method, or adding a payment method, if you do not have one listed there. .
Here is where to log-in to your Apple ID account.
Apple ID Log-in:
https://appleid.apple.com/.
If needed:
Below are articles online (from Apple Support) to help you:
Change, Add or Remove Apple ID Payment methods
https://support.apple.com/en-us/HT201266
Payment methods that you can use with your Apple ID:
(Scroll down to find your country)
https://support.apple.com/en-au/HT202631
OR
Here ia Apple's Billing webpage:
https://support.apple.com/en-au/billing
Hope this helps you get back in charge of your app downloads :)