Compare a single value entity reference field to a multi-value entity reference field
$Article_Countries = $article-&gt;field_article_countries-&gt;getString();

This returns a string

-&gt;condition('field_user_countries', $Article_Countries, 'IN')

From the docs, condition with IN accepts an array.

So, you need $Article_Countries to be an array of entity IDs. My first thought was $article-&gt;field_article_countries-&gt;referencedEntities() to get an array of objects, and then foreach them to build an array of IDs, but that's a bit round-the-houses. So I searched for a neater way and found this: http://drupal.stackexchange.com/a/207715/89581 which suggests:

$ids = array_column($entity-&gt;field-&gt;getValue(), 'target_id');