make sure that you set the failure redirect in the server validation serve behavior to go back to the checkout page.
to show failure error messages, you need to apply the validation show if behaviors.
To retain validated data, you need to set the initial values of the form elements to use the validated entries dynamic binding collection.
see the validation toolkit online help:
look in the Getting Started guide -> Tutorials -> Server Validation section under:
Displaying server validation errors
Maintaining form data when validations fail