As a follow up to this issue, I called support team for smartlook assistance, An expert helped me to delete S8 form from the back end, he also tried many times and spend more than one hour to delete it, Now I still can't deleted an unwanted form by myself, the expert does not know the cause of the issue,  maybe he just forced to deleted forms from back end, there is no fundamental solution to this issue, it means I still need to call to support team to ask an expert to deleted forms if I face the same issue next time. It's weird, customers should able to delete forms on their own, I hope PRO TAX team can solve this problem as soon as possible,  may be this is a bug of Pro tax