Error:NotFound message on ChronoForms 6 for WP

I'm trying to add Chronoforms to a WordPress site. I have the form created, and everything SHOULD work (I used a similar form on several Joomla sites). However, I get an error message when I try to submit it - "Error: NotFound". At first I thought is was related to ReCaptcha, but it persisted even after I removed the ReCatpcha from the form.