You have deleted the field with the calculated amount. That's why you are getting an invalid amount configuration error when you try to submit the form. Login to Neartail > click on the form to open it > Edit page will be displayed > In the Neartail Edit page, click on the Add price field icon > select Order amount, click Next > select Standard calculation using TOTAL() formula, click Next > click Create. You can then submit a response and test the form.