If Auto Ordering fails with "Payment authentication failed," "Authentication error," or "Invalid login credentials," the problem is almost always the fulfillment (Amazon) account, not the specific order.
The most common causes are:
2FA on the Amazon account isn't set up the way Easync needs it (it must be app-based only — set up with the 2FA setup code, not a QR code, and with no SMS/phone method, per help.easync.io/en/articles/2279678), the Amazon password was recently changed and no longer matches the one saved in Easync under Fulfillment Accounts, or Amazon has temporarily locked the account.
A second, separate cause looks similar but comes from eBay: if your buyer hasn't actually paid yet, Easync can't order — set "require immediate payment" on your EASYNC_PAYMENT_PROFILE in eBay so this stops happening.
Keep in mind Easync does not retry a failed order on its own — you have to click the "make order" basket button to retry.
What to do:
open the order
click Details
read the exact error in the red banner
confirm your Amazon login and 2FA are correct in Fulfillment Accounts and that the buyer has paid
then retry
If it still fails, send support the Easync order link (one per order) and they'll check the account and reprocess it — for account-level 2FA/password issues, fixing that first is what lets the stuck orders and their tracking go through.