Check the network connection:
You need to have a good Service and/or Wi-Fi.
If they are On, turn Off Hotspot & VPN.
Charge the battery > 70%.
If you can't connect to the App Store, iTunes Store, or other Apple services
If you can't connect to the App Store, iTunes Store, or other Apple services - Apple Support
👉 If you tried all of the above steps and you still can’t connect, contact Apple Support.
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The try again to add funds to your Apple Account balance, see -> Add money to your Apple Account balance
Add money to your Apple Account balance - Apple Support
See also the following Apple articles:
Add a payment method to your Apple ID
Add a payment method to your Apple ID – Apple Support (UK)
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.
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.