Why visitors may see an error when leaving a review
When someone submits a review, you are notified (go to Settings → Notifications to manage these alerts). By default, new reviews are set to hidden from the website until you approve them.
If there are reviews in the system but none are public, the next visitor who tries to leave a review may see an error message. This happens because the system detects existing reviews, but there are no visible reviews to display.
How to fix this
To prevent this error:
Approve the review so it appears on your website, or
Delete the hidden review if you don’t wish to publish it.
As a rule of thumb: if your only review is hidden, make sure it is either approved or deleted to avoid errors for future visitors.