This message relates to the invoice number that has already been used in AFAS, which prevents the booking from being processed.
To resolve this you need to adjust several settings in AFAS:
Go in AFAS to General / Setup / Number settings / Counters
Select the correct journal and period
Click the tab 'Ranges'
Double-click the range
Enter the last invoice number in the field 'Current value'
The booking can now be exported again.
Please note: the AFAS invoice number is the document number in Zenvoices. Please note this article for more information.