The problem occurs because when registering a form in the payment gateway, the URL configured as a webhook is also used to redirect the user after completing the process. However, a webhook is designed to automatically receive data from the payment gateway server and not to be visited directly by the user.
For example, if you configure a webhook with a tool like Buildship, when the user is redirected to that URL, instead of processing the information as a server would do, it simply displays the error message you see in the image.