Rules / Build

LaTeX Error: Can be used only in preamble — Preamble command in the document body

A command such as \usepackage is used after \begin{document}.

Fix

Move it above \begin{document}.

Example

Reproduced on TeX Live 2026 by CI · pdflatex · corpus/usepackage-after-begin-document

\usepackage after \begin{document}

Fails: main.tex
\documentclass{article}
\begin{document}
\usepackage{booktabs}
\begin{tabular}{ll}\toprule
  A & B \\ \bottomrule
\end{tabular}
\end{document}
Fixed
\documentclass{article}
\usepackage{booktabs}
\begin{document}
\begin{tabular}{ll}\toprule
  A & B \\ \bottomrule
\end{tabular}
\end{document}
What TeX prints in main.log
! LaTeX Error: Can be used only in preamble.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.3 \usepackage
               {booktabs}
Your command was ignored.
Type  I <command> <return>  to replace it with another command,
or  <return>  to continue without it.

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