To pay for a subscription (a payment that comes back every month or every year) without a credit card, use an App Store or Google Play gift card. You buy one at a grocery store, a pharmacy or a convenience store.
Buy a gift card from the right store: App Store for iPhone, Google Play for Android.
Scratch the code on the back.
In the app store, tap your profile picture, then Redeem Gift Card or Redeem code.
Type the code. You should see the amount added to your balance.
If you don’t see it: check that the card comes from the right store and that the code is correctly copied. Your purchase receipt lets you have the card checked in store.
Limitation: when the balance is empty, the subscription stops or looks for another payment method. Some stores also accept a debit card or PayPal (an online payment service).
To picture it: a gift card works like pocket money kept in your phone.
Good to know: no legitimate organization asks you to pay a bill with gift cards. That request is a sign of fraud: do not pay, hang up, then call the organization at the number you already know.