A PHP Error was encountered - Message: Undefined index: title

The only place in the controller code you show, that references an index "title", is

Code:

$data['title'] = $data['post']['title'];

This must be line 59, no?

The error message suggests that $data['post'], which you get from

Code:

$this->Post_model->get_posts($slug);

is not returning an array with a 'title' element.

This sounds like the problem lies in your Post_model, or in the datait is retrieving.