activate maintenance mode wordpress

function activate_maintenance_mode() {

 if ( !(current_user_can( 'administrator' ) || current_user_can( 'super admin' ))) {

 wp_die('<h1> Sorry Site Closed </h1>

<p>Hello، 
Site Under maintenance.

 Come back after a few minutes.
 
 <br />
 
 <strong> 
 Thank you for your understanding
 !
 </strong>
 
 </p>', 'Sorry Site Closed');
 }
}
add_action('get_header', 'activate_maintenance_mode');