\documentclass{article}
\usepackage{tikz}
\begin{document}
Lorem ipsum \ldots
\end{document}

(./pgfcore.code.tex

LaTeX Warning: You have requested package `pgfcore',
               but the package provides `['.

Package: [ 2019/01/05 v3.1 (3.1)

! LaTeX Error: Missing \begin{document}.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...

l.10 \ProvidesPackageRCS[v
                          \pgfversion] $Header: /cvsroot/pgf/pgf/generic/pgf...

You're in trouble here.  Try typing  <return>  to proceed.
If that doesn't work, type  X <return>  to quit.

LaTeX Warning: You have requested package `structure-mod', but the package provides `structure'.
Package biblatex Warning: 'babel' option is deprecated. Please use 'autolang' instead.
Package hyperref Warning: Option `backref' has already been used, setting the option has no effect on input line 947.
Package hyperref Warning: Option `pagebackref' has already been used, setting the option has no effect on input line 947.
Package hyperref Warning: Option `hyperindex' has already been used, setting the option has no effect on input line 947.

This is pdfTeX, Version 3.14159265-2.6-1.40.19 (MiKTeX 2.9.6930)
entering extended mode
("C:/Users/utente/Desktop/Fondamenti fisica Libro/Libro Fondamenti /Main
-mod.tex"
LaTeX2e <2018-12-01>
("C:\Program Files\MiKTeX 2.9\tex/latex/base\book.cls"
Document Class: book 2018/09/03 v1.4i Standard LaTeX document class
("C:\Program Files\MiKTeX 2.9\tex/latex/base\bk12.clo"))
("C:/Users/utente/Desktop/Fondamenti fisica Libro/Libro Fondamenti\stru
cture-mod.sty"

LaTeX Warning: You have requested package `structure-mod',

LaTeX Warning: Label `fig:subimppp' multiply defined.


LaTeX Warning: Label `fig:subimcicloide' multiply defined.


LaTeX Warning: Label `fig:submirage' multiply defined.


LaTeX Warning: Label `la'' multiply defined

%----------------------------------------------------------------------------------
%	SECTION NUMBERING IN THE MARGIN
%----------------------------------------------------------------------------------


\renewcommand{\@seccntformat}[1]{\llap{\textcolor{RedOrange}{\csname the#1\endcsname}\hspace{.35em}}}
\renewcommand{\section}{\@startsection{section}{1}{\z@}
{-4ex \@plus -1ex \@minus -.4ex}
{1ex \@plus.2ex }
{\hspace{2em}\normalfont\large\sffamily\bfseries}}
\renewcommand{\subsection}{\@startsection {subsection}{2}{\z@}