So you decided to use formspree so you can submit forms with your static pages, but you’re getting a
400 Bad request error, and it may come with “could not find the “Referrer” header.”.
This simply means that formspree expects to receive a header called “referrer”, which webservers tend to include, however, if you’re testing this with static HTML, it’s not a webserver and contains no referrer header, so formspree complains. The same applied to a jekyll site running on my local environment with
jekyll serve, but it worked fine when I uploaded the website to the server.
Put those files on a webserver, if it fails on your local machine, try putting it on the live server.