Query for a custom post taxonomy

Please try this modification of your code:

$args = array( 
        'post_type' => 'immobili',
        'tax_query' => array(
                array(
                        'taxonomy' => 'immobili_categories',
                        'field' => 'slug',
                        'terms' => 'realizzazioni',
                ),
        ),
);


// The Query
$query = new WP_Query( $args );