Sorry but I could not do that. I don't know how your payment gateway works, so I could not check and tell you what's wrong
You can ask me any technical questions related to payment plugin development, but it must be a general question and not specific to a payment gateway.
To develop payment gateway, you will have to read their developer document and develop it yourself. The only way I can help is reading their development document to understand what data they pass back to your website when payment completed, how to validate and make sure the payment is valid, but I cannot afford to do that, it will take much time
BTW, what's the error you are having? You can attach a screenshot of that error page so that I can take a quick look to see whether I knows why the error happens and hopefully, can guide you where to check