You should edit the invoice template.
Go to 'Custom form styles' from settings and edit the invoice.
You can add the due date from contents menu and also preview the pdf to verify if it is added.
You should also verify that the template you are editing is set as default so that you have the one when creating an invoice.
If you need more details for editing of template please reply to the post to comment on the issues you have.
Cheers.
