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.