Autotask PSA: QuickBooks Online: Error '400 Duplicate Document Number'



The sync of an invoice from Autotask to QuickBooks Online is failing, and 'ERROR' is appearing within the Autotask Invoice Number field. You see the following output in AppConnect:

"Error creating the QuickBooks invoice for Autotask invoice ID _____ and account _____. Reason: There was an error. 400 Duplicate Document Number Error Duplicate Document Number Error : You must specify a different number. This number has already been used. The invoice number for Autotask invoice id _____ and account _____', was set to 'ERROR' in Autotask."


  • Autotask PSA


The custom transaction numbering option is enabled in QuickBooks Online, and the application is having difficulty assigning the 'next' invoice number because it's colliding with a number that is already in use.


To correct this behavior, turn off custom transaction numbering in QuickBooks Online by performing the following steps:

  1. In QuickBooks Online, select the tools icon in the top right-hand corner.
  2. Select Accounts and Settings.
  3. Select Sales from the left-hand menu.
  4. Edit the Sales form content section.
  5. Disable the Custom transaction numbers setting.
  6. Attempt the sync operation again.

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