This error usually occurs because the transaction (Invoice) that is trying to sync to QuickBooks Online (QBO) is dated in a period that is already closed in QBO.
However, it can also happen if something on the invoice was added to a previously synced invoice and has since been updated.
Either way, the best way to resolve this is to do the following in your QuickBooks account:
Go to Settings ⚙, then select Accounts and Settings.
Proceed to the Advanced tab.
Select Edit ✎ in the Accounting section.
Uncheck the Close the books box or change the closing date.
If you need to change the password, just enter a new password and confirm it.
Select Save and then Done.
I believe they may have changed it from a checkbox to a toggle on some accounts.
Turn the toggle off, then retry the invoice sync in DF, then turn the toggle back on.
This should resolve this sync error.
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.