post_type needs to be max. 20 characters, can not contain capital letters or spaces, in register_post_type(); function your post type name is capitalize Portfolio replace with lower letters portfolio
this is my register post type snippet, follow it
// add action register our post type portfolio
add_action( 'init', 'register_cpt_portfolio' );
// Register our Custom Post type as portfolio
function register_cpt_portfolio() {
// labels text for our post type portfolio
$labels = array(
// post type general name
'name' => __( 'Portfolio' ),
// post type singular name
'singular_name' => __( 'Portfolio Item' ),
'add_new' => __( 'Add New Portfolio Item' ),
'add_new_item' => __( 'Add New Portfolio Item' ),
'edit_item' => __( 'Edit Portfolio Item' ),
'new_item' => __( 'New Portfolio Item' ),
'view_item' => __( 'View Portfolio Item' ),
'search_items' => __( 'Search Portfolio Items' ),
'not_found' => __( 'No Portfolio Items found' ),
'not_found_in_trash' => __( 'No Portfolio Items found in Trash' ),
'parent_item_colon' => '',
'menu_name' => 'Portfolio'
);
$args = array(
'labels' => $labels,
'public' => true,
'publicly_queryable' => true,
'show_ui' => true,
'query_var' => true,
'capability_type' => 'post',
'hierarchical' => false,
'has_archive' => true,
'menu_position' => 5,
'supports' => array( 'title', 'editor', 'thumbnail', 'author', ),
'rewrite' => array( 'slug' => 'portfolio-item', 'with_front' => false )
);
register_post_type( 'portfolio' , $args );
}