Most probably this error happened because of passing an invalid cart_id format. For example, passing integer id instead of a string.

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 the "API debug log", by navigating to the Developer’s menu > Testing > API Debug Logs (within your desired profile).

Or you may get the error directly from the response payload sent from PayTabs according to your sent request

Reason #1: Invalid cart_id format

This means that you passed an invalid format to the "cart_id" parameter, such as integer values or symbols.

The cart_id format MUST be string, to know more about the format and maximum length please check our Request/Response Parameters | Cart ID (cart_id) : PayTabs Support Portal solution article.