By default, all Invoices have the Buyer as the Department. However, you can change the default Department of a Supplier so that all Invoices will be automatically assigned when it comes into Purchase Plus. Please check out this article for more information Set a default Department for a Supplier.
If the Invoice hasn't picked the default Department assigned to them, check either of the below options,
If the invoice came in before the Department was set up, the Department will need to be set manually as the change is not retroactive and will only apply to the invoices coming in after the change has been made.
If the Invoice has come in after the default has been changed and the Invoice still didn't pick up the Department, please check the audit log for the history of the Invoice and double-check the Supplier has been set with a default Department, via this link Set a default Department for a Supplier.
(Note - If there is a Purchase Order that can be matched to the invoices, Purchase Plus will pick up the Department and Account code values from the Purchase Order).