The subscribe widget is not showing on a product page

The widget checks whether the product has any selling plans and renders nothing when it does not. Almost every report comes down to one of these four, in this order:

1. The product is not attached to an offer. This is by far the most common cause. Open Offers and check the product is listed under one. A product with no offer has no selling plans, so there is nothing for the widget to show.
2. The offer is attached to different variants. If you picked specific variants rather than the whole product, only those variants are subscribable. A variant added after the offer was created is not included until you edit the offer.
3. The block is not on this template. Themes can have several product templates. The block has to be added to the one this product uses, not just to the default.
4. The block was added to a different theme. Theme edits apply per theme. Check you customized the theme that is actually published.