Runtime exception page not found (modular form; action route)

I am testing a contact form in a modular page.

The form fields display as expected, and I have followed the details at

https://learn.getgrav.org/17/forms/forms/how-to-forms-in-modular-pages

to double check I am doing it correctly. In particular

form:
    action: /home

where 'home' is the page route for the modular page.

When I attempt to send a message, there is a

 RuntimeException (400)
Display page not found. Please check the page exists.

(with reference to the form plugin).

When I attempt to view the modular page directly, using example.com/home I receive a

404 page not found error

Anyone have a suggestion for what I might be missing, or need to do differently?

Thanks!