Shopify did not accept one or more of this product's variants because they have no SKU. It can happen on a first transfer or on a later update.
Why it happens
Shopify needs a SKU on every variant when stock is kept at a location run by a fulfillment service. For new products, that is the location chosen in Settings → Products → General, under Shopify inventory location. A product already in Shopify keeps the location where its variants are stocked.
A variant reaches Shopify without a SKU when:
The Etsy listing, or some of its variations, has no SKU on Etsy.
In Settings → Products → Etsy → Shopify, under Product Variations, Variation SKUs is set to Leave empty.
SKU generation is on, but the listing has two options and its quantity varies for only one of them, so Shuttle does not generate SKUs for it.
How to fix it
In Settings → Products → Etsy → Shopify, under Product Variations, change Variation SKUs from Leave empty to Use Etsy variation SKU and click Save.
On Etsy, give every variation of the listing a SKU. This also covers the two-option listings above.
To let Shuttle create the SKUs instead, go to Settings → Products → General. Under Automatically generate SKUs, check Enable SKU generation, fill in the Template field and click Save. See Understanding How Shuttle's SKU Template Works.
Then go to Settings → Products → Etsy → Shopify. Under SKUs, check Automatically generate SKUs from Etsy to Shopify and click Save. Shuttle also adds the new SKUs to your Etsy listing.
Transfer or update the product again.
For a new product, you can also pick your own location under Shopify inventory location.
Still seeing it?
If every variation has a SKU on Etsy and the transfer or update still fails, contact support with the product's name.