This error occurs when an invoice in DesignFiles has been updated or created and is attempting to sync to QuickBooks Online, but the customer on the invoice has been deleted in QBO.
You can resolve this one of two ways:
Edit the invoice
Select an active customer (if the wrong one was initially selected in error)
In QuickBooks Online
Reactivate the associated customer that was deleted
To reactivate a customer in QBO:
Select Sales or Invoicing from the left menu.
Select Customers at the top.
At the top of the customer list, click the small gear and select Include Inactive.
Click the customer you wish to restore.
Click the Make active button next to the customer name.
If you have followed all the steps and you are still experiencing the same sync error, please contact us via the Chat Bubble in the bottom right.