Etsy did not accept the product because one of its variation names was longer than 45 characters.
Why it happens
Etsy allows up to 45 characters for each variation name, such as "Size" or "Color". The limit is for the name only, not for the choices under it, such as "Small" or "Red".
When you transfer a product from Shopify, Shuttle shortens any long name before sending it, so a long Shopify name does not cause this error. It comes from the listing on Etsy instead.
When you update a listing that is already on Etsy, Shuttle sends the listing's own variation names. If two of them share a name, such as two called "Size", Shuttle tells them apart by adding the scale, like "Size (Inches)", or a number, like "Size #1". That longer name can go past 45 characters, and Etsy rejects it.
Shortening the name in Shopify does not help here, because the names come from the Etsy listing.
How to fix it
In Etsy, open the listing and look at its variations. Find two that share the same name.
Give them different names, each 45 characters or fewer.
In Shuttle, go to Products, find the product, click Update and choose Update in Etsy.
See Updating products with Shuttle if you need the full update steps.
Still seeing it?
Contact support with the product name if the listing's variation names are all different and 45 characters or fewer, and the error still shows.