The cause:
This message comes up if you've tried to export one or more Pleo invoices, but no fiscal year for invoices has been set up in Datev.
The solution:
An admin or your tax consultant can create a fiscal year for invoices in DUO. Please note that a requirement is to be on the Extended (Erweitert) version of DUO.
Here's where to check if a fiscal year set up: