Most probably this error happened due to a typo or wrong transaction class within the payment request.

to get the best out of this article, check first the article What is the "tran_class" (transaction class)?

In this article you will be going to know about:

How to investigate the reason that caused this error?

You can investigate the reason for such an error from your Paytabs dashboard, 

Developers > API DEBUG LOGS

Also, You can see in the response error message

Reason #1: The payment request contains a wrong transaction class

This means that the transaction class (tran_class) within your payment request, either is wrong (not valid) or you may have a typo as the above figure.

Make sure to use one of the valid transaction classes within the payment request. For more details about the transaction class parameter kindly check the article Request/Response Parameters | Transaction Class (tran_class)