Resolve Checkout Payment Error

Every time a customer reaches the final step of my website’s checkout, an error message pops up and the transaction is aborted. No charge is captured and no order record is created, so I’m losing sales in real time... (Budget: ₹600 - ₹1500 INR, Jobs: API Integration, Debugging, MySQL, Payment Gateway Integration, PayPal API, PHP, Software Architecture, Web Development)