understanding wp_next_scheduled

You’re probably attempting to add this too early. Wait for init.

add_action(
    'init',
    function () {
        if ( ! wp_next_scheduled( 'send_booking_expiration_hook' ) ) {
            wp_schedule_event( strtotime( '12pm' ), 'hourly', 'send_booking_expiration_hook' );
        }
    }
);