Fix LaTeX Error: Environment bmatrix undefined Error – Latex Tutorial

Like undefined multirow error, this error is also missing latex package.

To fix this error, you should use \usepackage{amsmath}

Here is an example:

\documentclass[preprint,12pt]{elsarticle}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{float}