I'm unable to install new apps or update downloaded ones

You may need to have a valid payment method on file, even if the app you want to download is free.

You can have a credit card / debit card on file and an Apple Account balance.

Your default payment method will be used first by Apple when you need to make a payment.  

If you have set your Apple Account balance as your default payment method and you have a CC / debit card, you pay from your default payment method.

See -> If you can’t download or update apps on your iPhone or iPad

If you can’t download or update apps on your iPhone or iPad – Apple Support (UK)

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Add a payment method

You may need to have a valid payment method on file, even if the app you want to download is free.

Add a payment method

• If you want to change your card, add first the new card, then remove the old one.
• or if you want to use your Apple ID balance, you need to set your Apple ID balance as your default payment.

See also -> Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

fyi: Not all credit & debit cards seem to work with App Store / iTunes Store, try a different one, or preferably a credit card. If your card is not accepted as a paymenf method, you need to contact your bank.

Check also -> 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

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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 still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service or check -> here for more support and assistance.