How to pass Post_ID variable from theme’s Single.php to custom Plugin using AJAX

I may be missing something in your code; but:

1. you need to “move” the content of your test div to a JS variable that can then be used in your Ajax (or use the div value directly in your $.post.

   e.g. var postID = $('#test').val();

2. then use it to pass the needed info to your server

   $.post( checkbox.ajaxurl, {
      action : 'submit_checkboxes',
      nonce : checkbox.nonce,
      thispost : postID,
      post : $(this).serialize()
     },
     function(response) {whatever... }
   );
   

3. in your PHP use say intval($_POST['thispost']) to obtain the post id and use accordingly.

Note: not tested & I tend to use jQuery.ajax with a data string of values instead of jQuery.post