The Finanzamt asks for an answer or documents by a date
A Frist is the time within which you must do something, for example reply or file a Widerspruch.
What you can do
Reply in time and include proof if you have any.
If you cannot get a document, tell the office in time. Explain why.
Hand in copies and keep the originals. Write down what you handed in and when.
If you do not reply, the office decides without your explanation.