This means a transfer stopped partway, and Shuttle did not get a reason it could show you. It appears on Etsy orders going to Shopify and on Shopify products sent to Etsy automatically.
Why it happens
Shopify refused the order or did not answer. Shopify turned down a new Etsy order, or a change to one already in Shopify, with no clear reason, or the connection dropped before it replied. This is usually short-lived.
An automatic transfer hit an unexpected problem. When Shuttle sends a new Etsy order or Shopify product on its own and something goes wrong partway, you see this same message. Shuttle could not tell which side caused it.
How to fix it
Wait a few minutes, then retry the way that matches what failed.
Etsy order that never reached Shopify
If the order was going as a draft, check your Shopify draft orders first so you do not create a second copy.
Go to Orders, find the order and click Transfer.
Choose Normal order or Draft order, then click Transfer as order or Transfer as draft.
Order in Shopify that did not update
This bulk run needs a paid plan or the free 14-day trial. On the free plan, Transfer or update orders shows as Unavailable.
Go to Orders and click Transfer Orders.
Choose Transfer or update orders, then Only update existing.
Under Which orders to process?, choose By date range, pick dates that include the order, and click Transfer now.
Shopify product that did not reach Etsy
Go to Products, find the product and click Transfer to Etsy.
In the window that opens, click Transfer product.
Still seeing it?
If it keeps failing over a few hours, contact support with the Etsy order number or product title.