Etsy refused this product because at least one of its tags has a character Etsy does not accept.
Why it happens
Etsy tags may only use letters, numbers, spaces, hyphens and apostrophes. When Shuttle transfers or updates a product, it removes symbols and emoji from every tag first. It leaves out any tag it cannot clean.
Shuttle does keep some letters with accents, such as é, ü or ç, and the letter ß. If Etsy refuses one of these, you see this error. The tag can come from:
the product's own tags in Shopify
the tags you entered under Add custom tags in Shuttle, including the product type, vendor or category that the [TYPE], [VENDOR] and [CATEGORY] placeholders add
the Shopify metafield you set in Shuttle for tags
How to fix it
In Shopify, open the product and look at its tags. Change each accented letter to the plain letter. For example, "café" becomes "cafe". Save the product.
In Shuttle, go to Settings → Products → Shopify → Etsy and find the Tags section.
Check the tags under Add custom tags in the same way. If they use [TYPE], [VENDOR] or [CATEGORY], also check the product's type, vendor and category in Shopify.
If the Shopify metafield key for tags field is filled in, open that metafield on the product in Shopify and check the tags in it.
Save your changes, then transfer the product again.
Still seeing it?
If the product still fails after every tag uses only plain letters, numbers, spaces, hyphens and apostrophes, contact support and include the product's name.