This message means one or more option names on the product were too long for Etsy. Shuttle used a shorter name on Etsy, and the product still transferred.
Etsy accepts option names of up to 45 characters. Shuttle shortens a long name, usually at the last whole word that fits. Your Shopify product keeps its full option name. If a shortened name matches another option name on the product, the shortened one gets " #2" at the end on Etsy.
Why it happens
An option name in Shopify is longer than 45 characters. Check the option names on the Shopify product. A long question used as an option name is a common example.
Two options were merged into one. If Enable option merging is on and the Shopify product has 3 options, Shuttle joins two option names with the Merge separator and a space on each side, for example "Size & Material". Each name can fit on its own while the joined name does not. See Transferring Shopify Products with 3 Options to Etsy.
How to fix it
You do not have to do anything. The listing works with the shorter name. To choose the Etsy name yourself:
In Shopify, open the product and rename any option longer than 45 characters to 45 characters or less.
If the product has 3 options, go to Settings > Products > Shopify → Etsy and find Merge options for products with 3 options.
If Enable option merging is on, check Exclude from merge and Merge separator to see which two options get merged.
Shorten those two option names in Shopify so that both names, the separator and 2 spaces add up to 45 characters or less. Or change Exclude from merge to merge a different pair.
Shuttle uses the new names the next time you transfer the product.
Still seeing it?
Contact support if the next transfer still shows this message although every option name, and any merged pair with its separator and 2 spaces, is 45 characters or less. Send the product name.