get kurs by coach

// WP_Query arguments
$args = array(
	'post_type'              => array( 'kurs' ),
	'meta_query'             => array(
		array(
			'key'     => 'coach_of_the_post',
			'value'   => '$coach->slug',
			'compare' => 'LIKE',
		),
	),
);

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