payment gateway not working magento 2.1.9

Hi Guys,
I m working with magento2.1.9 on my project. but i am stuck with payment gateway(
As i place order from frontend on checkout, its giving me error:
An error occur on the server.Please try to place your order again.
If i look into the backedn, the order is placed successfully with “Pending” Status.
If i look into debug.log file, the response i got from
‘x_response_reason_code’ => ‘99’,
‘x_response_reason_text’ => ‘This transaction cannot be accepted.’,

I am not getting, where i am doing wrong.Please help me out.