The thing that pops out is that you are using get_the_terms to get the taxonomy terms attached to the post and then using wp_get_post_terms to effectively do the same thing (they are very similar).
Without reproducing your set up I can't test this, but whenever I'm writing some code such as this I liberally add error_log messages throughout the code so I can see whether a function returns what is expected. A typical problem may be that you are expecting a function to return an object, but it returns an array of objects.
So, make sure your debug log is on.
If you haven't already edit your wp-config.php file and change the line with WP_DEBUG like so:
define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);
That will create a file debug.log in your /wp-content/ directory which you can examine in any text editor.
Then update your code with error log messages, something like this:
// Manage Voyage Country Taxonomies
add_action('cred_save_data', 'cred_save_voyage_countries', 10, 2);
function cred_save_voyage_countries($post_id, $form_data) {
$forms = [ 90, 110 ];
if ( in_array( $form_data['id'], $forms ) )
{
error_log("starting...");
$embarkationcountry = get_the_terms( $post_id, 'embarkation-country' );
$disembarkationcountry = get_the_terms( $post_id, 'disembarkation-country' );
error_log(print_r($embarkationcountry, true)); //should be array of term objects
error_log(print_r($disembarkationcountry, true)); //should be array of term objects
if ( !empty( $embarkationcountry ) ){
$emb_term_list = wp_get_post_terms($post_id, 'embarkation-country', array("fields" => "all"));
$emb_term_slug = $emb_term_list[0]->slug;
$country_slug_emb = get_term_by('slug', $emb_term_slug, 'countries');
$country_id_emb = $country_slug_emb->term_id;
error_log(print_r($emb_term_list, true));
error_log(print_r($emb_term_slug, true));
error_log(print_r($country_slug_emb, true));
error_log(print_r($country_id_emb, true));
wp_set_post_terms($post_id, $country_id_emb, 'countries');
}
if ( !empty( $disembarkationcountry ) ){
$dis_term_list = wp_get_post_terms($post_id, 'disembarkation-country', array("fields" => "all"));
$dis_term_slug = $dis_term_list[0]->slug;
$country_slug_dis = get_term_by('slug', $country_slug_dis, 'countries');
$country_id_dis = $country_slug_dis->term_id;
wp_set_post_terms($post_id, $country_id_dis, 'countries');
}
}
}
Note how I'm using print_r inside the error_log messages for variables, because anything that isn't simple text (e.g. an array) will be formatted more helpfully.
You should be able to see from the messages in debug.log whether at each step what is being returned and used is what is expected.
(This is a crude alternative to running a fully-fledged PHP IDE for debugging.)
Try that and if you are still stuck let me know.
