Downloading app
Why can’t I download a free app without a debit card?
iPhone 8, iOS 12
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See Add funds to your Apple ID https://support.apple.com/en-us/HT208687
"Before you can add funds to your Apple ID, you must have a valid payment method on file. Learn about the payment methods that you can use with your Apple ID. ... See how to Add funds to your Apple ID https://support.apple.com/en-us/HT208687 Note that "after you authorize the payment, the funds ... [are credited] to your Apple ID. Then, you can use the credit to make purchases from the App Store or iTunes store. Need more help? Contact Apple support for help here: Get started."
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Also see Create or use an Apple ID without a payment method https://support.apple.com/en-us/HT204034