Error - 'Unable to create purchase invoices. The Invoice number has already been used'

This issue occurs when trying to submit an expense with a reference that has already been used.

Check the supplier record to see if the reference has already been used. If so, amend the reference within the download expense screen to one that hasn't been used yet.

To do this:

Click on the expense
Select Edit Header Information
Change the reference
Click Save

Create a Purchase Order that will then be delivered and invoiced to the Purchase Ledger

Unmatch an invoice that has already been matched to a purchase order

Error - 'Invoice x has had detail entered and cannot be deleted'

Deleting a nominal code if it has not been used?

Error - 'The invoice has been cancelled, so cannot be matched.' when matching invoice to PO