Sometimes when a business starts to use Unify to sync transactions to QuickBooks, they are catching up with periods of time that have since past and have been closed automatically by QuickBooks online based on their preferences.
If a user needs to enter a large amount of transactions for historic periods, it's common to temporarily turn off the closing of books in QuickBooks online. This can then be re-enabled when historic periods have been fully posted and reconciled, so don't worry!
To post orders from a period of your books that you have already closed, simply log in to QuickBooks Online. Navigate to your Account Settings area under the gear icon at the top right of QuickBooks Online.
Within that screen, click on Advanced Settings. In the below screenshot, you will see the field where you have saved a closed date for the period. Delete that close date and save, assuring that the Close Books feature is off. Now you should be able to return to Unify and post transactions from previous periods.