Routing issue with $routes->post

Are you sure the form is posted to the right URL? Show your opening form tag.

I'm using it as an API end point. Testing it with postman. Sending a post request.

It doesn't matter if it's an html form or if you use postman. At what URL do you post the data? If CI doesn't find the route, maybe that route isn't configured at all or is misconfigured or you have an error in your post url.