Some form elements were added/removed/modified after updating the template in the 'Form to Email' page.
You may go to the 'Form to email' page and make sure that all the required form elements are present in the template.
This warning occurs in the following sequence:
Go back to the 'Form to email' page and press the 'Auto-generate' button.
Alternatively, if you don't want to use the default template, go to the 'Form to email' page and make sure that the template is updated according to the elements present in the form.
For example, if you had added some form elements, add the new element to the template.