User gets an error: "The account period has closed" while syncing invoices with QuickBooks Online.
This can happen because of the company's closing date. You can check for the Closing Date in CORE in the Company Profile screen. If the date is okay there, check the closing date of QuickBooks Online to see if it has been set to a recent date. You can change this date to match it with the transaction date of the invoice. That should resolve the issue.