LaTeX Error: Environment align* undefined

This error will appear when LaTeX is unable to understand the command you may have used.

doc.tex, line 5
LaTeX Error: Environment equation* undefined.
See the LaTeX manual or LaTeX Companion for explanation. Type H for immediate help. ... l.5 \begin{equation*} Your command was ignored. Type I to replace it with another command, or to continue without it.

Common Causes of Error
Forgot to load a package
When you are trying to make use of an environment which is part of a specific package, and you forget to load the package in your preamble, an error will occur. For example, the align environment. The align environment is a part of amsmath package. So, if you want to use the align environment, it is necessary to include \usepackage{amsmath} in the preamble.

% Preamble

\usepackage{amsmath}

% In your main.tex file

\begin{align}
5x + 7y &= 8\
6x - 4y &= 9
\end{align}

In the case when \usepackage{amsmath} is not included in your preamble, an error message will be generated.

You have spelled the environment incorrectly
Another reason for this error is when you spell the environment incorrectly. For example,

\begin{lisY}
\item First list item
\item Second list item
\end{lisY}

The above lines will throw an error because there is a spelling mistake as instead of list, lisY is written. To avoid this, check the spelling while using environments.