Understanding - how to payment prompt

Hallo, in our system we want customers only to pay via bank-transfer.
In case ot that, i dont neet a payment gateway.

Now i generated invoices, ok.
But if the customer has not paid, i want to send a payment prompt (in German Mahnung or Erinnerung)
In The system i open the invoice, press the button “make” and can take “Zahlungsaufforderung” - i think, this is, what i want.(?)
But after that it opens a window with the message, that no payment gateway is configured.

So - how - and where to configure, that this is only a document (i can print or make pdf) to give it to the customer?
Like - "Dear Customer, you have already not payd our invoice … "

In this case, i want to know, how to set up “Mahngebühren” - overdue fines?

Thanks a lot for help