Root cause: the Commerce sellable item has not been published.

Solution

  1. Check whether you can see the product in "Business Tools".
  2. If it is there, open the product and check whether it is published in the "Entity Versions" section.
  3. If the sellable item is associated with a workflow, its status may show as false. In the Sitecore Content tree, open the Workflow tab, find the sellable item, and approve it. The error should then disappear.
Sitecore Commerce Entity Versions publication status

Let me know which step resolved the issue.