LaTeX Error: Something's wrong--perhaps a missing \item - Unsure, why?

Question

What am I doing wrong? (I am very new to LaTeX, and this is my 1st question, pls forgive my ignorance)

   \section*{TERM 2} 
    \textbf{10 weeks} (30 hrs) 
        \begin{itemize}
            \subsection*{UNIT 1: Number \& Algebra (9 wks)} 
                \subsubsection*{Topic 1.2: Real Numbers}
                    \begin{itemize}
                      \item \underline{Week 1:} Sub-Topic 5
                      \item \underline{Week 2:} Sub-Topic 6
                      \item \underline{Week 3:} Sub-Topic 7
                      \item \underline{Week 4:} Sub-Topic 8
                      \item \underline{Week 5:} Sub-Topic 9
                      \item \underline{Week 6:} Sub-Topic 10
                      \item \underline{Week 7:} Sub-Topic 11
                      \item \underline{Week 8:} Sub-Topic 12
                    \end{itemize}

The error comes up at "sub-topic 5" (line 7). I'm using Overleaf, and this bit is part of a 100-page doc. This same error comes up four times. It displays well on PDF, though. I appreciate any help you can provide.

Answer 1

I think those sections subsections etc. are to change font size and shape. The standard itemize environment is not very useful. Therefore, we need to load enumitem for the extra options.

I am not sure how exactly you want the list to look like, so I added a few things like bold, italics, different fontsizes etc. and hopefully the following code will be a good start for further experimenting.

\documentclass{article}
\usepackage{enumitem}

%%% Sets global keys
\SetEnumitemKey{cancelfont}{
    before=\normalfont\normalsize}
\SetEnumitemKey{customlabel}{label={}}

%%% Sets formatting per each level of itemize
\setlist[itemize,1]{
    customlabel,
    cancelfont, before*=\LARGE\bfseries}
\setlist[itemize,2]{
    customlabel,
    cancelfont, before*=\large\bfseries\slshape}
\setlist[itemize,3]{
    customlabel,
    cancelfont, before*=\bfseries}
\setlist[itemize,4]{customlabel, cancelfont}



%%% Document
\begin{document}

\noindent
\begin{itemize}
    \item TERM 1
    \item 10 weeks (30 hrs)
    \begin{itemize}
        \item UNIT 1: Number \& Algebra (9 wks)
        \begin{itemize}
            \item Topic 1.2: Real Numbers
            \begin{itemize}
                \item \underline{Week 1:} Sub-Topic 5
                \item \underline{Week 2:} Sub-Topic 6
                \item \underline{Week 3:} Sub-Topic 7
                \item \underline{Week 4:} Sub-Topic 8
                \item \underline{Week 5:} Sub-Topic 9
                \item \underline{Week 6:} Sub-Topic 10
                \item \underline{Week 7:} Sub-Topic 11
                \item \underline{Week 8:} Sub-Topic 12
            \end{itemize}
        \end{itemize}
        \item UNIT 2: Number \& Algebra (9 wks)
        \begin{itemize}
            \item Topic 1.2: Real Numbers
            \begin{itemize}
                \item \underline{Week 1:} Sub-Topic 5
                \item \underline{Week 2:} Sub-Topic 6
                \item \underline{Week 3:} Sub-Topic 7
                \item \underline{Week 4:} Sub-Topic 8
                \item \underline{Week 5:} Sub-Topic 9
                \item \underline{Week 6:} Sub-Topic 10
                \item \underline{Week 7:} Sub-Topic 11
                \item \underline{Week 8:} Sub-Topic 12
            \end{itemize}
        \end{itemize}
    \end{itemize}
    \item TERM 2
    \item 10 weeks (30 hrs)
    \begin{itemize}
        \item UNIT 3: Number \& Algebra (9 wks)
        \begin{itemize}
            \item Topic 1.2: Real Numbers
            \begin{itemize}
                \item \underline{Week 1:} Sub-Topic 5
                \item \underline{Week 2:} Sub-Topic 6
                \item \underline{Week 3:} Sub-Topic 7
                \item \underline{Week 4:} Sub-Topic 8
                \item \underline{Week 5:} Sub-Topic 9
                \item \underline{Week 6:} Sub-Topic 10
                \item \underline{Week 7:} Sub-Topic 11
                \item \underline{Week 8:} Sub-Topic 12
            \end{itemize}
        \end{itemize}
    \end{itemize}
\end{itemize}

                
\end{document}

Answer 2

You combine two different types of structure:

1. outline (sections, subsections, subsubsections)
2. (nested) lists

A (nested) list should be completely inside one level of your outline, for example

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\begin{document}
   \section{abc}
   \begin{itemize}
      \item a
      \item b
      \item c
   \end{itemize}
   \subsection{def}
   \begin{itemize}
      \item d
      \item e
      \item f
   \end{itemize}
\end{document}