Notify Author of the post if admin deletes his post and perform some function

Firstly, it’s better to user WP functions than write your own SQL. Even though it might be a bit more expensive:

$post = get_post($post_ID);
$authorid = $post->post_author;

Secondly, it’s not get_usermeta(), but get_user_meta() and the complete call should include true as the third parameter:

$currentPointNumber = get_user_meta($authorid, 'points', true);

And then it’s update_user_meta(), again, and you don’t need to initialize a new variable, just do:

update_user_meta($authorid, 'points', $currentPointNumber-1);