CS forms - simple contact form 404 error

Hello there
I’m pretty sure, I did everything what Josh said here, but the component part (not necessary):
https://www.youtube.com/watch?v=9uq0SMeFKY8

as soon as I hit the submit button, I get 404 and no form submission is saved in the back end “form submissions” part.

What am I missing?

EDIT: wp forms plugin works.