// paste below code in your functions.php

function cmb2_get_post_options( $query_args ) {

	$args = wp_parse_args( $query_args, array(
		'post_type'   => 'post',
		'numberposts' => 10,
	) );

	$posts = get_posts( $args );

	$post_options = array();
	if ( $posts ) {
		foreach ( $posts as $post ) {
          $post_options[ $post->ID ] = $post->post_title;
		}
	}

	return $post_options;
}


function cmb2_get_your_post_type_post_options() {
	return cmb2_get_post_options( array( 'post_type' => 'your_post_type', 'numberposts' => 5 ) );
}





// and this given below post paste in your meta data file

$cmb_demo->add_field( array(
	'name'       => __( 'Select your_post_type Posts', 'cmb2' ),
	'desc'       => __( 'field description (optional)', 'cmb2' ),
	'id'         => $prefix . 'post_multicheckbox',
	'type'       => 'multicheck',
	'options_cb' => 'cmb2_get_your_post_type_post_options',
) );