The product is not published to your Headless storefront's sales channel, or not sold in the visitor's market.
The symptom. The template page opens, but the product block stays empty, or the product is missing after the CMS sync.
Why. Your site reads Shopify through the Headless storefront. Shopify answers only for products that are Active and published to that storefront’s sales channel, in the visitor’s market. A product published only to the Online Store does not exist for your site.
The fix. In Shopify, open the product, and in the Publishing card add your Headless storefront. Do the same for its collections, and check the markets it is sold in. Then sync the CMS again.
To show a message instead of an empty block, such as “not available in your country”, use condition="product-unavailable".