[Payments App] No Backend Requests Received From Shopify At Checkout

[Prashu1999]

Hi
I have developed a payment app on shopify
My app is approved and published and properly installed on shopify store

I have enabled the test mode and during checkout I am getting this error (Your payment can’t be processed for technical reasons. Try again or use a different payment method.)
After doing some research I realized that I am not receiving any backend request from shopify as they have stated in (https://shopify.dev/apps/payments/processing-a-payment#how-the-payments-app-flow-works ) that  after the customer completes checkout, it will trigger a request for payment.
I have checked the access log and got nothing in the log
I also tried using my own custom log but the result is same
do anyone have any idea related to this