If you get this message when trying to sync invoices from ServiceBox to Sage, please try disabling fast posting in Sage. To do this follow the following steps:

To turn on or off Fast Posting

  1. In the Home window, open the Setup menu and select Settings.
  2. On the left side of the window, click Company and then System.
  3. Clear the Fast Posting option and click OK.


Source: Turn On or Off Fast Posting (sage.com)