"Pay with Amazon Express" error help in PHP Laravel

I have integrated "Pay with Amazon Express" in my PHP Laravel Project It works fine in native PHP but in Laravel it shows following error "

We're sorry, but there's a problem processing your payment from this website.

Your account has not been charged. Please contact seller for assistance."

Can you help me that how can I debug it because I am unable to find what thing is causing error . Or what is causing this error?

I have used the following code http://ift.tt/2uh0neh and error snapshot is as follows: screen shot of error screen

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