带图形的抽认卡

Question

我想制作一些解剖笔记的抽认卡，我想我需要使用课堂flashcards。但我的问题是抽认卡上不能有图形/图像，它不会编译；它给出了错误

“未定义控制序列”

我究竟做错了什么？

\documentclass[avery5388,grid,frame]{flashcards}

\cardfrontstyle[\large\slshape]{headings}
\cardbackstyle{empty}

\begin{document}

\cardfrontfoot{Functional Analysis}

\begin{flashcard}[Definition]{Lever celler}
  \includegraphics[width=.8\linewidth]{billeder/0604}
 \end{flashcard}


\begin{flashcard}[Definition]{Inner Product}

Let $X$ be a complex linear space. An \emph{inner product} on $X$ is
a mapping that associates to each pair of vectors $x$, $y$ a scalar,
denoted $(x,y)$, that satisfies the following properties:

\medskip

\begin{description}
\item [Additivity] $(x+y,z) = (x,z) + (y,z)$,
\item [Homogeneity] $(\alpha \: x, y) = \alpha (x,y)$,
\item [Symmetry] $(x,y) = \overline{(y,x)}$,
\item [Positive Definiteness] $(x,x) > 0$, when $x\neq0$.
\end{description}
\end{flashcard}
    \end{document}

谢谢