How to plot the commutative triangle diagram in Tikz?

Use the correct syntax for the arrows. I propose this code:

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

\begin{document}

\begin{tikzcd}[row sep=4em, column sep=2em]
    A \arrow[rr, "f"] \arrow[swap, dr, "g"] & & B \arrow[dl, "h"] \\
    & C \\[-4.6em]
 & \overset{ \rotatebox{90}{=}}{D}
\end{tikzcd}

\end{document} 

I add also my proposal:

\documentclass[a4paper,12pt]{article}
\usepackage{mathtools,amssymb}
\usepackage{tikz-cd}


\begin{document}
\begin{tikzcd}[row sep= 1cm, column sep=.6cm]
A \arrow[rd, "g"'] \arrow[rr, "f"] & & B \arrow[ld, "h"] \\
& \underset{\textstyle{D}}{\overset{\textstyle{C}}{\shortparallel}} &                  
\end{tikzcd}

\end{document}