$this->session->sess_destroy(); error

What do you mean why not? You can't arbitrarily make-up a rule and then ask why it is not true.

- The documentation says it's loaded via $this->load->library() (and NOT $this->load->driver()).
- The database "library" also has drivers, and it's not loaded via $autoload['drivers'].
- The inline documentation (comments in the autoload.php file itself) above $autoload['drivers']) says "they extend the CI_Driver_Library class"; the Session library does not extend that class.

That last one has been explained in other topics around here too.