What if I don’t have a payment method

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.

If you have set your Apple Account balance as your default payment method, your default payment method will be used first by Apple when you need to make a payment.

Some purchases might require you to have a credit card on file, even if you use your Apple Account balance to make the purchase.

Then you can’t download or update apps on your iPhone or iPad

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)