How to get node editing form in a custom module
15 is the node ID of the node whose edit form you want, not the actual ID of the form. This makes sense since node edit forms are defined per nodetype, not per node. 

You'll need to call drupal_get_form() with the form ID, something like [nodetype_name]_node_form.

The second parameter should be the node object, e.g. node_load(15).