Fatal error: Call to undefined function register_rest_route()

//The Following registers an api route with multiple parameters. 

 <?php
   add_action( 'rest_api_init', 'add_custom_users_api');

    function add_custom_users_api()
    {
        register_rest_route( 'route', array(
                                    'methods' => 'GET',
                                    'callback' => 'function',
                           ));
    }