Etsy rejected the product because one or more of its tags are longer than Etsy allows. Etsy's limit is 20 characters per tag.
Why it happens
Shuttle normally leaves out tags that are too long before it sends a product to Etsy. Sometimes a tag that Shuttle lets through is still too long for Etsy, and Etsy then rejects the whole product.
Shuttle takes tags from up to three places. Check each one you use:
Your Shopify tags, if Transfer Shopify tags is turned on.
Your custom tags, which Shuttle adds to every listing. A placeholder can make a short tag long, because [TYPE], [VENDOR] and [CATEGORY] are replaced by the product's type, vendor and category.
Your tags metafield, if you entered a Shopify metafield key for tags.
How to fix it
In Shopify, open the product and shorten any tag longer than 20 characters.
In Shuttle, go to Settings > Products > Shopify → Etsy and scroll to Tags.
Under Add custom tags, make each custom tag 20 characters or fewer. Count the length after the placeholder is filled in for this product.
If you use Tags from metafield, open the product in Shopify and shorten the long tags in that metafield.
To keep a long tag in Shopify but stop sending it to Etsy, add it to Exclude tags.
Save, then transfer or update the product again.
For more on these settings, see Managing Product Tags During Transfers and Updates.
Still seeing it?
If every tag from all of these places is 20 characters or fewer and Etsy still rejects the product, contact support and tell us which product it is.