For any in-app purchase, you must add a valid payment method to your Google account.
You can always add, remove or edit your payment method.

Edit a payment method
1. On your Android device, open the [Google Play Store] app.
2. Select [Menu] > [Payment methods] > [More payment settings]
3. Sign in to [pay.google.com]
4. Select [Edit] under the payment method you want to edit
5. Make your changes and tap [Update]

In order to add or remove payment methods, please check the link below:
Add, remove or edit your payment method