natbib can operate in two citation modes:
authoryear (the default) and
You can only use all of
natbib's features if you use a
\bibliographystyle. Specifically, your style must export the author list and year of the current work in the optional argument of
\bibitem. (Some more details can be found in Package natbib Error: Bibliography not compatible with author-year citations (again!).)
If your style is not
natbib compatible in this sense, you will still be able to use the style if you load
natbib with the
numbers option and do not use advanced features like
\citeauthor. The full feature set (
\citeauthor, ...) will not be available in that case though.
Your template loads
% Festlegung Art der Zitierung - Havardmethode: Abkuerzung Autor + Jahr
which despite the comment would not normally be called a "Harvard style" and is not compatible with
With that setting you can only use
numbers mode with reduced functionality.
If you want
authoryear to work, you need to load a different style. If you have no particular requirements
one of the base
natbib styles might suffice.
Note that you may have to delete the
.bbl files after you change your bibliography style, so LaTeX and BibTeX can pick everything up correctly. Otherwise old settings may stay around in these temporary files and can cause error messages that are no longer relevant.
Unless you somehow have to use this particular template, you might want to look for a slightly simpler template elsewhere (or no template at all! just start with a very basic document and load the stuff you need yourself). The code is strewn across so many different files in different folder levels that it is really hard to find out what is going on. The template also does a few things one should not do (it combine a KOMA class with
hyperref comparably early, forces a lot of
\newpages when a
scrrprt-like class would start chapters on new pages automatically, wields many
\addcontentsline when other more elegant solutions exist, ...).