This problem is caused by a combination of two misconfigurations. First, AJAX JavaScript is not functioning on your contact form. Due to this problem, your contact form needlessly redirects after submission. Second, your contact form uses unavailable words in the names of input fields. This issue confuses WordPress, resulting in the 404 (“Not Found”) error.

https://contactform7.com/faq/are-there-any-reserved-or-unavailable-words-for-the-name-of-an-input-field/

https://contactform7.com/why-isnt-my-ajax-contact-form-working-correctly/