Publishing failed. The response is not a valid JSON response in WordPress.

 

Publishing failed. The response is not a valid JSON response in WordPress.

 

When you create a function for shortcode you should must be return value.
 
function my_shortcode_function() {
ob_start();  
?>
<p>Hello World!</p>
<?php
return ob_get_clean();
}add_shortcode('shortcode','my_shortcode_function');