Well, the error you get results in the following wrong code (environment cvitems is not defined):
\begin{cvitems} % Description(s) of tasks/responsibilities
\item {an item}
\end{cvitems}
Change it to
\begin{itemize} % <===============================================
\item {an item}
\end{itemize} % <=================================================
The following MWE compiles without error
\documentclass[11pt,a4paper,sans]{moderncv}
% moderncv themes
\moderncvstyle{classic} % casual, classic, banking, oldstyle and fancy
\moderncvcolor{blue}
\usepackage[utf8]{inputenc}
\usepackage[scale=0.75]{geometry}
\name{John}{Doe}
\title{Resumé title}
\address{street and number}{postcode city}{country}
\phone[mobile]{+1~(234)~567~890}
\phone[fixed]{+2~(345)~678~901}
\phone[fax]{+3~(456)~789~012}
\email{john@doe.org}
\homepage{www.johndoe.com}
\social[linkedin]{john.doe}
\social[twitter]{jdoe}
\social[github]{jdoe}
\extrainfo{additional information}
\photo[64pt][0.4pt]{example-image-a}
\quote{Some quote}
\setlength{\footskip}{66pt}
\begin{document}
\makecvtitle
\section{Education}
\cventry{year--year}{Degree}{Institution--3}{City--4}{\textit{Grade}--5}{Description--6} % arguments 3 to 6 can be left empty
\cventry{year--year}{Degree}{Institution}{City}{\textit{Grade}}{Description}
\cventry
{Journalist} % Job title
{Company} % Organization
{Indiana} % Location
{2015 - Present} % Date(s)
{---THIS IS AN OPTIONAL TEXT---}
{
\begin{itemize} % <===============================================
\item {an item}
\end{itemize} % <=================================================
}
\section{Master thesis}
\cvitem{title}{\emph{Title}}
\cvitem{supervisors}{Supervisors}
\cvitem{description}{Short thesis abstract}
\section{Experience}
\subsection{Vocational}
\cventry{year--year}{Job title}{Employer}{City}{}{General description no longer than 1--2 lines.\newline{}%
Detailed achievements:%
\begin{itemize}%
\item Achievement 1;
\item Achievement 2, with sub-achievements:
\begin{itemize}%
\item Sub-achievement (a);
\item Sub-achievement (b), with sub-sub-achievements (don't do this!);
\begin{itemize}
\item Sub-sub-achievement i;
\item Sub-sub-achievement ii;
\item Sub-sub-achievement iii;
\end{itemize}
\item Sub-achievement (c);
\end{itemize}
\item Achievement 3.
\end{itemize}}
\cventry{year--year}{Job title}{Employer}{City}{}{Description line 1\newline{}Description line 2}
\subsection{Miscellaneous}
\cventry{year--year}{Job title}{Employer}{City}{}{Description}
\section{Languages}
\cvitemwithcomment{Language 1}{Skill level}{Comment}
\cvitemwithcomment{\textbf{Language} 2}{\textbf{Skill} level}{Comment}
\cvitemwithcomment{Language 3}{Skill level}{Comment}
\section{Computer skills}
\cvdoubleitem{category 1}{XXX, YYY, ZZZ}{category 4}{XXX, YYY, ZZZ}
\cvdoubleitem{category 2}{XXX, YYY, ZZZ}{category 5}{XXX, YYY, ZZZ}
\cvdoubleitem{category 3}{XXX, YYY, ZZZ}{category 6}{XXX, YYY, ZZZ}
\section{Interests}
\cvitem{hobby 1}{Description}
\cvitem{hobby 2}{Description}
\cvitem{hobby 3}{Description}
\section{Extra 1}
\cvlistitem{Item 1}
\cvlistitem{Item 2}
\cvlistitem{Item 3. This item is particularly long and therefore normally spans over several lines. Did you notice the indentation when the line wraps?}
\section{Extra 2}
\cvlistdoubleitem{Item 1}{Item 4}
\cvlistdoubleitem{Item 2}{Item 5}
\cvlistdoubleitem{Item 3}{Item 6. Like item 3 in the single column list before, this item is particularly long to wrap over several lines.}
\section{References}
\begin{cvcolumns}
\cvcolumn{Category 1}{\begin{itemize}\item Person 1\item Person 2\item Person 3\end{itemize}}
\cvcolumn{Category 2}{Amongst others:\begin{itemize}\item Person 1, and\item Person 2\end{itemize}(more upon request)}
\cvcolumn[0.5]{All the rest \& some more}{\textit{That} person, and \textbf{those} also (all available upon request).}
\end{cvcolumns}
\clearpage
%----- letter ---------------------------------------------------------
% recipient data
\recipient{Company Recruitment team}{Company, Inc.\\123 somestreet\\some city}
\date{January 01, 1984}
\opening{Dear Sir or Madam,}
\closing{Yours faithfully,}
\enclosure[Attached]{curriculum vit\ae{}} % use an optional argument to use a string other than "Enclosure", or redefine \enclname
\makelettertitle
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arcu, sed tempor quam. Curabitur pharetra tincidunt tincidunt. Morbi
volutpat feugiat mauris, quis tempor neque vehicula volutpat. Duis
tristique justo vel massa fermentum accumsan. Mauris ante elit, feugiat
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odio id odio aliquam mattis. Donec sapien nulla, feugiat eget adipiscing
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Albert Einstein discovered that $e=mc^2$ in 1905.
\[ e=\lim_{n \to \infty} \left(1+\frac{1}{n}\right)^n \]
\makeletterclosing
\end{document}
and results in

If you want to have the 5th parameter without text just do not write one:
\cventry
{Journalist} % Job title
{Company} % Organization
{Indiana} % Location
{2015 - Present} % Date(s)
{} % <========================================= just leave it empty
{
\begin{itemize} % <===============================================
\item {an item}
\end{itemize} % <=================================================
}
\cventry{1}{2}{3}{4}{}{6}when it's not there, or do you want to have\cventry{1}{2}{3}{4}[5]{6}when it is there (note the[]v{})? – Teepeemm Aug 14 '17 at 22:20