Error : This feature is not available for the database you are using
hmmm , try with
$this->db->query("CALL update_letter_body(".$body_letter.",".$insert_id.")");call_function enables you to call PHP database functions that are not natively included in CodeIgniter,not for calling procedures you've written.
check the documentation
https://www.codeigniter.com/user_guide/database/call_function.html