Parent and Child relation for custom post types

To elaborate on what Mayeenul Islam said, from the WP Docs on regsiter_post_type

hierarchical
(boolean) (optional) Whether the post type is hierarchical (e.g.
page). Allows Parent to be specified. The ‘supports’ parameter should
contain ‘page-attributes’ to show the parent select box on the editor
page. Default: false

Emphasis mine. After setting that to true, you should be able to associate the posts with each other.

EDIT: Code added

$support = array (
    'title',
    'editor',
    'author',
    'thumbnail',
    'custom-fields',
    'comments',
    'genesis-seo',
    'genesis-layouts',
    'revisions',
    // Add this to supports
    'page-attributes',
);

$args = array  (
    'labels' => $labels,
    'public' => TRUE,
    'rewrite' => array('slug'=>('produkte-leistungen'),'with_front'=>false),
    'capability_type' => 'page',
    // You had this set to FALSE, try it with TRUE
    'hierarchical' => TRUE,
    'query_var' => true,
    'supports' => $support,
    'taxonomies' => array('portfolio-category'),
    'menu_position' => 5
);

register_post_type('portfolio',$args);

In the above snippet, I added ‘page-attributes’ to $support and set the ‘hierarchical’ option to TRUE in $args. That should get you set up properly.

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