The cause:
One or more Pleo expenses have a date that falls within a closed accounting period.
There are two alternative solutions:
If the affected expense is a Pocket expense, you can manually change the date in Pleo so that it falls within an open accounting period.
If the affected expense is a card expense, open the closed accounting period in Quickbooks:
Go to "Account and Settings".
Click "Advanced Settings".
Un-tick the box that says "Closing the books".
Click "Save".