If you receive a message when syncing an invoice to QuickBooks Online that says "Error syncing invoice: Validation Exception was thrown: Details: The account period has closed and the account books cannot be updated through the QBO Services Api, please use the QBO website to make these changes", the invoice date is before the closing date set in QuickBooks Online (under Settings->Advanced->Close the books).
Setting a closing date in QuickBooks Online prevents us from importing invoices from before this date.
To fix this you have three options:
1. Turn off the 'Close the books' setting so that the system can import the invoice.
2. Move the 'Closing Date' to a earlier date before the invoices you want to import.
3. Manually create the invoices in QBO and move your 'Invoices on or after date' to match the closing date in ServiceBox->QuickBooks Online->Settings->Invoices.