Wordpress添加，更新和获取Post Meta无法正常工作

The below code is added into a page template file, when someone loads the page it will add the post meta if it didn't exist, it will update the post meta if it did exist. When I get the post meta and then return it, the outcome is "Array".

It leads me to believe the post meta is not being made or the get isn't working.

global $post;
if ( ! add_post_meta( $post->ID, 'offer_voucher_evergreen_deadline', '2018-11-16 13:00:00')) { 
    update_post_meta($post->ID, 'offer_voucher_evergreen_deadline', '2018-11-16 13:00:00');
}

$test = get_post_meta($post->ID, 'offer_voucher_evergreen_deadline', true);
echo $test;

Try This:

global $post;
$meta_value = get_post_meta($post->ID, 'offer_voucher_evergreen_deadline', true);
if( !empty($meta_value) ) {
   update_post_meta($post->ID, 'offer_voucher_evergreen_deadline', '2018-11-16 13:00:00');
}else {
   add_post_meta( $post->ID, 'offer_voucher_evergreen_deadline', '2018-11-16 13:00:00')
}

$test = get_post_meta($post->ID, 'offer_voucher_evergreen_deadline', true);
echo $test; //return $test;

Note: Use a 3'rd param in get_post_meta() function, which will return a single value

$test = get_post_meta($post->ID, 'offer_voucher_evergreen_deadline', true);
echo $test; //return $test;

See get_post_meta() function for more details.