If the user does not set up number sequences then the following message, or similar may appear.
To set up a number sequence, use the navigation panel to go to General Ledger/Maintenance/Voucher number series and click on the ‘Add’ button in the toolbar/ribbon.
(Click on the screenshot to enlarge).
The user can then link the voucher number series to the number series under Customer/Maintenance/Number sequences.
Read more about number sequences here.
(Click on the screenshot to enlarge.)