Confusing "missing \item" error from itemize

The error you encounter can also be generated by the following code:

\documentclass{article}
\begin{document}
\begin{itemize}[nosep]
\item my first item
\end{itemize}
\end{document}

As you've already discovered, the real source of the error is the failure to load the enumitem package. The enumitem package, and not the LaTeX kernel, defines the optional setting nosep.

Nevertheless, it may be instructive to trace why LaTeX would issue the following messages when running the MWE shown above:

! LaTeX Error: Something's wrong--perhaps a missing \item.

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

l.4 \item m
           y first item
?

If the enumitem package is not loaded, LaTeX requires the very first item [pun intended] after \begin{itemize} to be \item. Thus, because the material in square brackets is not preceded by an \item statement in the MWE, LaTeX gets confused and stops with an admittedly not particularly clear error message.

Incidentally, if one inserts an \item directive immediately before [nosep], i.e., if one changes the MWE to

\documentclass{article}
\begin{document}
\begin{itemize}\item[nosep]
\item my first item
\end{itemize}
\end{document}

LaTeX runs just fine and the word "nosep" shows up as the substitute for the default level-1 marker (which happens to be a text bullet):

Finally, as already noted, it's essential to load the enumitem package in order to get the expected behavior from the [nosep] option.