<meta itemprop="availability" content="{% IF GOODS_MOD_REST_VALUE>0 %}in_stock{% ELSE %}out_of_stock{% ENDIF %}">
замените на
<meta itemprop="availability" content="{% IF GOODS_MOD_REST_VALUE>0 %}http://schema.org/InStock{%ELSE %}http://schema.org/OutOfStock{% ENDIF %}">