Error when compiling chapters into main document

Question

For my thesis, I want to compile various tex docs as chapters. Unfortunately, when trying to link the various chapters, I receive the following error:

File: chapter_1/tex/chapter1 "!Missing number, treated as zero.}\section{introduction}
File: chapter_1/tex/chapter1 "!Illegal unit of measure (pt inserted).}\section{introduction}

Below a snippet of my LaTex code:

The main document i.e. the compiler of the various chapters:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\documentclass
[
    paper=a4, % choose page size e.g. a4 or b5 (common booklet size)
    fontsize=12pt,
    headings=big,
    parskip, %indent at paragrafs?
    numbers=noendperiod, % 2.3.1 vs 2.3.1. (no dot after the last chapter number)
    twoside=true, %choose two sided for a booklet style, semi leaves the margings the same
    headinclude,
    pagesize,
    appendixprefix=false,
    headsepline,
    toc=bibliography, % Bibliography appears in Table of Contents (without a number)
    version=last % Use latest version of the KOMA-Script
]{Thesis_Template}

% Add the configuration and packages
\input{definitions/configuration}
\input{definitions/packages_commands}
\input{definitions/hyphenation}

% ad hoc settings
\interfootnotelinepenalty=10000

%import packages that need to be imported separately
\usepackage[subpreambles=false]{standalone}
\usepackage{import}

%toggle for footnotes to continue over the chapters
\counterwithout{footnote}{chapter}

\flipbindingmargins

% ad hoc spacing for sections
\setlength\parskip{1em plus 0em minus 0.1em}

\usepackage{titlesec}
\titlespacing*{\section}{0pt}{1em plus 0em minus 0.1em}{1em plus 0em}
\titlespacing*{\subsection}{0pt}{1em plus 0em minus 0.1em}{.6em plus 0em}
\titlespacing*{\subsubsection}{0pt}{1em plus 0em minus 0.1em}{.6em plus 0em}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{document}
\frontmatter
\headerpage

%% Startmain content
\mainmatter
\chapter[Chapter 1]{Chapter 1 long title} \label{chapt 1}
%import actual chapter tex
\import{chapter_1/tex/}{chapter1}   

\end{document}

An exemplary code of a chapter:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\documentclass[a4paper,11pt]{article}
\usepackage{standalone} \standalonetrue %to allow it to be part of thesis
\usepackage{hyperref}
\usepackage[english]{babel}
\usepackage{fullpage}
\usepackage{fancyvrb}
\usepackage{amsmath}
\usepackage{amsbsy}
\usepackage{amssymb}
 \usepackage[flushleft]{threeparttable}
\usepackage{amsthm}
\usepackage{tikzscale}
\usepackage{graphicx}
\usepackage{eurosym}
\usepackage{bm}
\usepackage{bibentry}
\usepackage{natbib}
\usepackage{csquotes}
\usepackage{booktabs}
\usepackage[font=small]{caption}
\usepackage{nameref}
\usepackage{caption}
\usepackage[toc,page]{appendix}
\usepackage{tabularx}
\usepackage{subcaption}
\usepackage{subfiles}
\usepackage{mathrsfs}
\usepackage{standalone}
\usepackage{lscape}
\usepackage{textcomp}
\usepackage{setspace}
\usepackage{dcolumn}
\usepackage{cleveref}
\usepackage[nomessages]{fp}
\usepackage{ifthen}
\usepackage{tablefootnote}
%Settings
\tolerance=2000
\usepackage[usenames,dvipsnames]{color}
\hypersetup{
  colorlinks,
  citecolor=blue,
  linkcolor=blue,
  urlcolor=blue
  }

\usepackage{tikz}
\usepackage{pst-plot}
\usepackage{pst-func}
\usepackage{animate}
\usepackage{graphicx}
\usepackage[explicit]{titlesec}

%sections setup
\titleformat{\section}{\normalfont\bfseries}{\thesection.}{.5em}{\hyperlink{toc}{{\black{#1}}}}
\titleformat{\subsection}{\normalfont}{\thesubsection.}{.5em}{\hyperlink{toc}{\it{\black{#1}}}}
\titleformat{\subsubsection}{\normalfont}{\thesubsubsection.}{.5em}{\it{#1}}
\titlespacing*{\subsubsection}{0pt}{11pt}{0pt}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{document}

\ifstandalone

\title{\sc{Title}
\unskip\thanks{Thanks to ...}
}
\author{John Doe}
 }

\date{\today}

\maketitle

\vspace{-1.5em}

%%%%%%% ABSTRACT %%%%%%%
\begin{center}
\parbox[t]{.9\textwidth}{ 
\begin{small}
\singlespacing
Abstract text here.

\clearpage

\doublespacing
\fi

%%%%%%% Introduction %%%%%%%

\section{Introduction}
Introduction text here

\end{document}

I am using TeX Maker. I also made sure that all packages are up to date.

I read online in other posts that it might have something to do with the titlesec package but could not figure out a solution.

Many thanks for your support!

Answer 1

The \import command is based on the \input command, which basically pastes the file pointed to right at the spot of the \input/\import. That includes \documentclass and the rest of your preamble. Of course, that's not going to work!

Instead, you should use the subfiles package. See How to bring existing LaTeX files as chapters of thesis appendix?

I'm not entirely sure this is what's going on here, because I get the more descriptive error Can be used only in preamble. But then, I'm using a very clean minimal document, and with more configurations going on, things mix up easier.