The rental calendar is not appearing on the product page

The calendar is a theme app block. Check that it has been added to the product template you are actually viewing, that you are on the published theme rather than a preview, and that it was re-added after any theme change, since app blocks do not carry across when a theme is duplicated or replaced. Also confirm the product has been set up as a rental product in the app: a product the app does not know about has no dates to draw.