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ok thank you very much

hello to everyone and a happy month!

I'm trying to cite through my bibliography and in PDF output it is only showing "?". Why?
\documentclass[
parskip=half,
ngerman
]{scrreprt}
\usepackage{babel}
\usepackage[utf8]{inputenc}
\usepackage[sort,nonamebreak,sectionbib,square]{natbib}
\usepackage ...