Apple Pay does not approve or decline transactions. Approvals and declines are only from your bank. Apple only securely transmits encrypted data from your Apple Wallet to your bank. The bank then approves or declines the transaction. If your bank approved the transaction, the bank then sends funds directly to the merchant. Apple never touches your money.
Please contact the bank that issued the card used to complete your transaction. If you need to file a dispute, you’ll need to do so with the bank.