If adding your Amazon/Amazon Business account to Easync shows "Error" even though you can log in to Amazon manually, the cause is almost always the 2FA setup:
Easync requires 2FA to be app-based only — in your Amazon security settings there must be nothing other than the authenticator app method (no SMS/phone), or Easync will only show "2FA error"
Set up 2FA using the 2FA setup code, not the QR code, following this article, then enter those details in Easync.
Once the 2FA is app-only and entered correctly, the account connects.
Proxy/connection errors that persist after this are escalated by support to the tech team.