How to write our own api in WP API Plugin?

Hello,

I am working on a wordpress website.now want the app for that.I have install WP REST API plugin version 1.2.2 and write my own api function.

My API = https://192.168.1.8:8090/infinitydemo/wp-json/showposts?author=1

function showposts()
{
$author = $_GET[‘author’];
echo $author;
$posts_array = array();

$args = array(“post_type” => “post”, “orderby” => “date”, “order” => “DESC”, “post_status” => “publish”, “post_author” => $author);

$posts = new WP_Query($args);
if(!empty($posts)){
$post_array = array(get_the_title(), get_the_permalink(), get_the_date(), get_the_content(), wp_get_attachment_url(get_post_thumbnail_id()));
array_push($posts_array, $post_array);

$success = array(“Message” => Success,”Response” => $posts_array);
}else{
$success = array(“Message” => Error,”Response” => $posts_array); }
echo json_encode($success);

exit(0);
}

but it showing
output :
[{“code”:”json_no_route”,”message”:”No route was found matching the URL and request method”}]

How can I can correct this ?
waiting for reply