Why product recommendations do not showing up in my store?

The Recommendation algorithm uses sales data and product descriptions to show a mix of products that are similar or often bought together. Product recommendations become more accurate over time as new orders and product data become available.

If you enable “product recommendations” in our theme but can not see any products on carousel, this is most likely because currently you do not have more sales for the recommendation to work. The recommendation works based on sales history and its accuracy improves over time to provide more relevant results

Please read this shopify official article to know more in details. This is well explained article by Shopify