If your attempt to buy a subscription is met with a declined or unsuccessful payment, follow these steps:

• Verify your payment information. It's possible that your payment profile is deactivated. To reactivate it, submit your payment info. More details on how to do it: https://support.google.com/googlepay/answer/7644078?hl=en
• Fix credit and debit card issues. Check for problems with your card, such as expiration, the need for additional information, insufficient funds, or specific restrictions from your bank or card issuer.
• Try to pay with a different payment method.

For further information, refer to the detailed response provided by Google Play Help