MailChimp - Webflow form leads to 404 page on submission

I have followed Webflow’s video help file on signup form integration with MailChimp which unfortunately doesn’t demonstrate what happens once it’s in action in real life.

I have used MailChimp’s form action code and set to POST in Webflow as instructed. When pressing submit it goes to a MailChimp 404 page! Why is this?

Also, shouldn’t it go through a captcha process?

https://uk-medical-freedom.webflow.io/