Creating a custom endpoint for rest, I see the endpoint exists in the wp-json, but the request is returning 404

Question

There’s an issue in your code: The HTTP method is not valid because WP_REST_Server::READABLE is a class constant, hence it should not be wrapped in quotes:

register_rest_route( 'adventapi/v1', '/adventapi/', array(
//  'methods' => 'WP_REST_Server::READABLE', // wrong
//  'methods' => "WP_REST_Server::READABLE", // wrong
    'methods' => WP_REST_Server::READABLE,   // correct

    // ... other args.
) );

You should also ensure that you use the correct request/HTTP method as well as correct endpoint URL, e.g. https://example.com/wp-json/adventapi/v1/adventapi, when making requests to the API.