Solved. Thanks for your help John.
My final code on content-product.php
$categories = get_the_terms($post_id, 'product_cat');
if(get_field('eref-' . $categories[0]->slug . ''))
{
echo '<div><span>REF: ' . get_field('eref-' . $categories[0]->slug . '') . '</span></div>';
}