How to Fix a LaTeX Error at \begin{paracol}

An error reported at \begin{paracol} can have more than one cause. The two most common are that LaTeX does not know the paracol environment because its package was not loaded, or that the environment was started without its required column-count argument.

Use \usepackage{paracol} in the preamble, then write \begin{paracol}{2} for two columns. The example below is a complete document you can compile to check that the package and environment work.

A minimal working example

\documentclass{article}
\usepackage{paracol}

\begin{document}

\begin{paracol}{2}
This text is in the first column.

\switchcolumn
This text is in the second column.
\end{paracol}

\end{document}

The {2} tells paracol to create two columns. \switchcolumn moves the text that follows into the next column. The final \end{paracol} closes the environment. To use more columns, change the number; for example, use {3} for three columns.

Diagnose the exact error

Read the first LaTeX error in the log, not only the line number highlighted by your editor. An error is often detected where LaTeX can no longer continue, even if the underlying mistake is on an earlier line.

Error or symptom	Likely cause	What to check
Environment paracol undefined	The package has not been loaded, or the build is using a setup that does not include it.	Add \usepackage{paracol} before \begin{document}.
Missing number, treated as zero	The required column count is missing or malformed.	Use \begin{paracol}{2} with a positive integer in braces.
Error at \end{paracol} or a later environment	The environments may not be properly nested, or an earlier environment was not closed.	Check that every \begin{...} has the matching \end{...} in reverse nesting order.
Works in a small test but fails in a journal template	The template may have package, class, or layout interactions.	Test the package in the actual template and consult its instructions before changing the class or bundled files.

If the error says the environment is undefined, adding {2} alone will not solve it: LaTeX still needs the package that defines the environment. If the package is loaded but the count is absent, adding the count is the relevant fix.

Using paracol in an MDPI template

Put \usepackage{paracol} in the document preamble, alongside other package imports, rather than editing the MDPI class or its definition files. Whether the package is already available depends on the specific template version; do not assume that the class loads it automatically. If LaTeX reports that the package is already loaded, avoid loading it a second time and check the template’s existing preamble.

Also consider whether you need paracol at all. MDPI templates set the manuscript’s page and column layout themselves. For ordinary journal text, let the template manage that layout rather than wrapping the whole manuscript in paracol. The package is intended for side-by-side columns whose contents may need independent control, such as parallel text. A journal’s production format may differ from the layout used for submission, so follow the journal’s current author instructions.

Common mistakes

• Leaving out the column count: \begin{paracol} is incomplete. Write \begin{paracol}{2} or another intended column count.
• Loading the package in the wrong place: Package imports belong in the preamble, before \begin{document}.
• Closing the wrong environment: If you nest environments, close the innermost one first. For example, an itemize started inside paracol must end before paracol does.
• Using the wrong tool for simple columns: If you only need a conventional multi-column layout, another environment or the document class’s layout settings may be a better fit. Choose based on the layout behavior you need, not just the first error message.

When the minimal example compiles but your full document does not, add the relevant preamble and content back in stages. That helps isolate whether the problem is a missing package, a malformed paracol start, or an interaction with another environment or template setting.