You get the error "At least one line item is mandatory" when you click on line items extraction but there are no line items to display. 

You will not be allowed to export the document unless you add a line or switch back to Invoice total tab.

There are two options how to fix "At least one line item is mandatory" error:

TIP: To learn more about how line items work in Datamolino, click here. Please note that the format and quality of documents are very important for extracting line items.