I’ve managed to make it work because of @howdy_mcGee’s comment. I did not notice that I had to call the specific post type name of it. my code is now like this and it shows up!
function slide_add_post_meta_boxes() {
add_meta_box(
'pukkemukslideshow', // Unique ID
__( 'Slideshow', 'example' ), // Title
'slide_post_class_meta_box', // Callback function
'slideshow', // Admin page (or post type)
'advanced', // Context
'high' // Priority
);
}