Call to undefined function wp_set_object_terms()

I want the users to be able to add tags to any post. I have a child theme twentytwelve-child.

In content.php I have written below code snippet. As you can see taxonomy_exists() function runs fine, but when I add the line
wp_set_object_terms($post_id, $user_tags, ‘post_tag’, true );
it gives me an error “call to undefined function”.

Both the functions are from the same “taxonomy.php” file which means that file is included, so why am I getting this error for for wp_set_object_terms function?

Please help.

<?php if ( is_single() ) { 

 $action=$_REQUEST['action'];
 $user_tags=$_REQUEST['user_tags'];
 $post_id=get_the_ID();
 if($action!="" && $user_tags!=null)
{
echo taxonomy_exists('post_tag');
wp_set_object_terms($post_id, $user_tags, 'post_tag', true );
}

?>
<form  action="" method="POST" enctype="multipart/form-data">
    <input type="hidden" name="action" value="submit">
Add more products/services offered(use comma to separate items): <p><textarea rows="10" cols="80" name="user_tags"></textarea></p>
<Center><input style="font-size:12pt;" type="submit" value="Submit" name="add-tags"/></CENTER>
</form>
<?php } ?>