If your payment method fails for some reason and you receive a payment declined / failure payment message, there can be multiple reasons behind that. Such as mentioned:
Insufficient Fund: Check if you have sufficient funds in the account/card
Invalid card details: Ensure to enter valid card details like the Card number
Invalid OTP: Enter valid One-time password (OTP) received from the Bank
Session timeout error: If you have been idle on the payment page for some minutes, the page will timeout for security reasons
Country-wise restrictions / Ineligible for International payment: Please check with your bank if the card is eligible for international payment or there are any country wise restrictions