This error appears when you have forgotten to include an \item command. It can also appear from trying to use lists inside a table incorrectly.

Common Causes

Forgetting to write \item:

The most basic example of this error is where you have told LaTeX that you are going to be writing a list, but you have not labelled your list entries with \item tags. An example of this is shown below

\begin{itemize}
This is the first entry in our list
\end{itemize}

Here, you have told LaTeX that you are writing a list by using the itemize environment. The mistake that is made is that you haven't labelled your list entry with \item. This will generate the error shown below:

The correct way to write this list is:

\begin{itemize}
\item This is the first entry in our list
\end{itemize}

See here to learn more about how to make different types of lists in LaTeX.

Writing lists inside tables incorrectly:

This error will also appear if you try to number cells within a table using lists, as shown below.

\begin{tabular}{l}
  \begin{enumerate}
  \item This is item number one\\ \hline
  \item This is item number two
  \end{enumerate}
\end{tabular}

This is a mistake, as lists cannot be used to number table cells in this way. One way to solve the problem above would be to create a counter. This can be done by including

\newcounter{rownumbers}
\newcommand\rownumber{\stepcounter{rownumbers}\arabic{rownumbers}}

in the preamble to your document (see here). You can then write the above example as

\begin{tabular}{l r}
  \rownumber & This is item number one\\ \hline
  \rownumber & This is item number two
\end{tabular}

To learn more about how to create counters, check out our documentation.