http://support.authorize.net/authkb/index?page=content&id=A852&actp=LIST
Quote:
A valid fingerprint or Transaction Key is required for this transaction.
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Please make sure you copied all Auth.net AIM details properly into X-Payments payment configuration for Authorize.net AIM integration. Usually such errors happen due to something being put in a wrong field or copied partially (especially for long IDs and keys).