LaTeX Error: Environment \foo undefined

TeX Output

./example.tex:3: LaTeX Error: Environment unknownenv undefined.
l.3 \begin{unknownenv}

Explanation

LaTeX was asked to enter an environment (\begin{unknownenv}) that has not been declared via \newenvironment in the document, class, or any imported package.

How to Fix

1. Check for spelling errors in the environment name (e.g. align* vs align, tabular vs table).
2. If the environment belongs to a third-party package (e.g. tikzpicture from tikz, algorithm from algorithm2e, or lstlisting from listings), add \usepackage{...} to the preamble.
3. If creating a custom environment, declare it with \newenvironment{unknownenv}{<before>}{<after>}.