\documentclass{scrbook}
\usepackage{mwe}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{csquotes}
\usepackage[
backend=biber,
style=authoryear,
]{biblatex}
\addbibresource{biblatex-examples.bib}
\begin{document}
\blindtext
\footcite[S. 39][Vgl.]{aristotle:anima}
\printbibliography[heading=subbibintoc]
\end{document}! Undefinded control sequence.
<argument> \blx@tempa

