If an invoice gets status "Incorrect" you need to save it again to get an error message. Go to Administration > Invoicing > click on the incorrect invoice in the list > Click Save > Read the error message. If you cannot interpret the error message, it is automatically generated by our system, so send the error message to us for help.
The most common error message is that the country code is missing, which can be fixed by going to the customer profile that the invoice is associated with, clicking "edit" and adding the country in the customer's address information. Save the changes, then go back to the invoice and save the invoice again.