Hi partner,
Thanks for your detailed information.
According to your description, the case field "Case Type", "Category" and "Sub Category" should be set by default value "X", "Order" and "Request".
I wonder that why you set the field value with regarding case's field from your last screenshot.

1.If you just want the default text value, so you should remove the "Regarding case" and only left "X", "Order" and "Request" and then you could create the case successfully without related case.

2.If you need the Regarding case's field value to be default value of new case, so you could add conditions "Email regarding is not null" to filter emails without related cases.
Hope it helps.
Best Regards,
Leo