Q:

Undefined sequece error when use the math environment

When I am trying to code in the math mode, the compiling process is terminated under any environment. How may I solve it?
\begin{document}
Let $\mathbf{A}$ be the set of $n \in \mathbb{N}$ that 
equation
\end{document}

A:

As others have already commented, you almost certainly failed to load the amssymb package in the preamble of your document. (Aside: The amssymb package loads the amsfonts package automatically.) For sure, the following example, which loads the amssymb package, compiles without any problems.
\documentclass{article}
\usepackage{amssymb} % <-- important
\begin{document}
Let $\mathbf{A}$ be the set of $n\in\mathbb{N}$ such that  \dots
\end{document}

