Why product recommendations do not show up
If you enable "product recommendations" in one of our theme but cannot see any products, this is most likely because you currently do not have enough sales for the recommendation engine to work properly. The recommendation engine of Shopify is indeed based on sales history and its accuracy improves over time to provide more relevant results.
The exact algorithm is explained in Shopify official documentation. Especially, if you are a non-Plus merchant (any standard plan), then the recommendation engine is based exclusively on the order history. This means that if you have not made any orders yet, Shopify will not be able to provide accurate recommendations.
If you have indeed made a lot of orders but still do not see any recommendations, you will need to reach Shopify support directly so they can investigate the issue on their end.