Recommend Rules do not trigger when no recommendations exist - they apply only when the model returns results.
This means that while you can pin any item with a Recommend Rule, it will only be pinned if there are recommendations returned on the source item. If no recommendations exist, the rule will not apply.
To ensure something is displayed, you can pass fallbackParameters in your Recommend request.
Fallback parameters are supported by the following models: Frequently Bought Together, Related Items, Related Content, Trending Items and Looking Similar.
Note that rules will not trigger if the recommendations are fallback items.
Related Article: Why is my Recommend Rule not boosting items as expected?