Shuttle could not export this review because it no longer has the product the review belongs to.
Why it happens
Shuttle keeps a record of every Etsy listing it has stored reviews for. This error means that record is missing for the listing this review was left on. Usually the listing had already been removed from Etsy when Shuttle stored the review. To check:
On the Reviews page, find the review. Its row says "No product assigned" while its status still says "Assigned · Auto".
On Etsy, check whether the reviewed listing still exists.
How to fix it
This row has no Assign button, so first have Shuttle reload the review from Etsy:
In Shuttle, open Reviews and click Store reviews.
Choose Re-store all reviews and click Store reviews. Your existing reviews are updated, and no duplicates are created. You can close the window while it runs.
When it finishes, find the review again.
If it now says Not assigned, click Assign, search for the Shopify product, select it and click Assign to reviews.
If it now shows a product, open the menu (…) at the end of the row and click Export review.
If the reviewed listing was deleted on Etsy, re-storing cannot bring the product back, and Shuttle cannot export this review. To move it out of your list, open the menu (…) on its row and click Exclude review.
Still seeing it?
Contact support if the listing still exists on Etsy, but after re-storing the review still says "No product assigned" and "Assigned · Auto".