1
If you want to use that function everywhere, you need to be sure that:
$current_user is always defined as a WP_User object,
$current_user->ID is different from 0 (zero) - only for logged in users,
jQuery select2() function (plugin) exist, avoiding JS errors on some other pages (where Select 2 is not enabled). See: How can I check if a jQuery plugin is ...
1
There are some errors as get_post_meta() first argument need to be a defined product ID here (and in your code post_ID is not even dynamic variable and will not give anything)…
Try the following:
<?php
function check_gtin(){
if ( $gtin = get_post_meta(get_the_id(), 'gtin', true) ){
echo "gtin13:". $gtin;
} else {
...
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