Markup describes; it does not decide
Adding review structured data makes a page eligible for a rich result. It does not guarantee one. Search engines choose whether to show stars based on their own judgement about the page and the query, and eligibility can be withdrawn.
The rating has to be visible to a visitor
The rule that catches most stores: the reviews behind the markup must be readable on the same page. Marking up a rating that lives somewhere else — or nowhere — is a policy violation, and it is the sort that gets a whole site’s rich results turned off.
Do not borrow someone else’s stars
An app store score, a marketplace rating or an aggregate from a third-party site is not your page’s rating. It is tempting when you are starting out and have none of your own. Publish nothing rather than publish that.
Self-serving reviews have a narrow rule
Reviews of your own business on your own site are not eligible for rich results in the way reviews of a product you sell are. The distinction is between rating the shop and rating the thing the shop sells; keep them apart.
No reviews is a valid state
A new product with no reviews should emit no rating. Validators will flag the entity as incomplete. That is the correct outcome, and it clears itself the day a real review arrives. We run our own product page exactly this way.
Explore this workflow in Dejor.
See how best shopify review apps compared fits into your store, including setup steps and common questions.