Some HTML elements are not permitted to be used in the form template. You will be warned by this message when the config-validator detects uses of unavailable HTML elements.
A typical example of an unavailable HTML element is the
<form> element. Because Contact Form 7 moves the content of the form template into a
<form> element, and according to the HTML specification a
<form> element is not allowed to be nested inside another
<form> element, use of
<form> elements in the form template make the HTML markup invalid and can lead to severe issues.