Etsy accepts SKUs of up to 32 characters. At least one variant of this product has a longer SKU in Shopify, so Etsy rejected the product's variations.
Why it happens
One or more SKUs on the product in Shopify have more than 32 characters. Spaces, dashes and other symbols count.
The setting "Skip all SKUs longer than 32 characters" is off, which is the default.
On a transfer, Shuttle creates the listing on Etsy as a draft, then Etsy rejects the variations. The draft stays matched to your product, so your next run is an update, and it fails the same way until you shorten the SKU or turn the setting on.
Automatic updates leave the long SKU out and send the rest, but still record the error.
Shuttle never shortens SKUs for you. A shortened SKU would no longer match your inventory.
How to fix it
To keep your SKUs on Etsy, shorten them:
In Shopify, open the product and change each SKU longer than 32 characters to 32 characters or less.
In Shuttle, go to Products > Shopify → Etsy, update the product and choose "Select all fields".
To send the product without the long SKUs:
In Shuttle, go to Settings > Products > Shopify → Etsy.
In the SKUs section, under Skip invalid SKUs, tick "Skip all SKUs longer than 32 characters".
Click Save.
Go to Products > Shopify → Etsy, update the product and choose "Select all fields". Variants with a long SKU arrive on Etsy without a SKU.
After a failed transfer, "Select all fields" rebuilds the Etsy variations by itself. You do not need "Recreate Etsy inventory".
Still seeing it?
If every SKU is 32 characters or less and a new update still gives this error, contact support with the product's name.