Why the tikzcd command does not compile

Question

I try to make a diagram:

\[
\begin{tikzcd}[column sep=small]
                    &D^n \arrow{r}{e_\alpha^n}    &X\arrow{r} &X/(X-e_\beta^{n-1}(B^{n-1}))\\
S^{n-1}\arrow{r}{\cong} &\partial D^n \arrow{u}[hook] &D^{n-1} \arrow{u}[swap]{e_\beta^{n-1}} \arrow{r} &D^{n-1}/\partial D^{n-1} \arrow{u}[swap,dashed]{\cong} \arrow{r}{\cong} &S^{n-1}
\end{tikzcd}
\]

But when I compile, it simply says undefined control sequence referring to the line containing \end{tikzcd}, but does not give any reason. What is wrong?

P.S. I have included the packages

\usepackage{tikz}
\usetikzlibrary{matrix,arrows,decorations.pathmorphing}
\usepackage{tikz-cd}
\tikzset{commutative diagrams/.cd}

Answer 1

You had a syntax error in

\arrow{u}[hook]

it should be

\arrow[hook]{u}

Similarly (although this won't produce errors), instead of

\arrow{u}[swap,dashed]{\cong} 

you should use

\arrow[swap,dashed]{u}{\cong} 

otherwise, the dashed option is ignored. The code:

\documentclass{article}
\usepackage{tikz-cd}

\begin{document}

\[
\begin{tikzcd}[column sep=small]
& D^n \arrow{r}{e_\alpha^n} & X\arrow{r} & X/(X-e_\beta^{n-1}(B^{n-1})) \\
S^{n-1}\arrow{r}{\cong} & \partial D^n \arrow[hook]{u} & D^{n-1} \arrow{u}[swap]{e_\beta^{n-1}} \arrow{r} &D^{n-1}/\partial D^{n-1} \arrow[swap,dashed]{u}{\cong} \arrow{r}{\cong} &S^{n-1}
\end{tikzcd}
\]

\end{document}

If you have a recent version of tikz-cd (using PGF/TikZ version 3.0), you have a more comfortable syntax (using the quotes library):

\documentclass{article}
\usepackage{tikz-cd}

\begin{document}

\[
\begin{tikzcd}[column sep=small]
& D^n \arrow[r,"e_\alpha^n"] & X\arrow[r] & X/(X-e_\beta^{n-1}(B^{n-1})) \\
S^{n-1}\arrow[r,"\cong"] & \partial D^n \arrow[u,hook] & D^{n-1} \arrow[u,swap,"e_\beta^{n-1}"] \arrow[r] &D^{n-1}/\partial D^{n-1} \arrow[u,swap,dashed,"\cong"] \arrow[r,"\cong"] &S^{n-1}
\end{tikzcd}
\]

\end{document}