If you tried to buy using Google Pay as your payment method and the transaction failed, there are several reasons why.
The most common reasons are:
1. Problems with your Google account;
2. Your card information does not match the information recorded in your Google Pay profile;
3. Other issues related to the card issuer.
So, we recommend that you check the Google payments page for alerts, make sure your card has available funds and is enabled for online purchases.
If you do, you can always try to remove your payment method and re-enter it, or you can always choose to pay with another card saved in Google Wallet.