Undefined control sequence with hyperref

I get Undefined control sequence error when I use hyperref to refer to the desired section.
I want to make the section number refer to the section when I click on it at the same time i want the number of the section appears automaticly by ref . Like this:

documentclass{article}
usepackage{harvard}

usepackage{xcolor}
usepackage{graphicx}

usepackage{url}
usepackage{hyperref}
usepackage{xspace}

renewcommand{harvardurl}{URL:url}
graphicspath{./301/Alaa Ali Yousef/}

title {Example LaTeX quad     article for course 4ME301}
author{Alaa Ali Yousef   9105195789 [email protected]   Linnaeus University, Växjö }


begin{document}
    maketitle

begin{abstract}
    This assignment aims to make you familiar with the basic features
    of LaTeX which should come useful when preparing your own reports,
    and eventually your master thesis. You should try to replicate “as
    close as possible” this PDF document using LaTeX. This does not
    exclude making some changes as long as they are relevant. Submission
    instructions can be found in Section ref{secondSection}.
    end{abstract}

section{Introduction section}
label{firstsection}
There is a handy book about LaTeX {color{green}cite{lamport1994latex}} available at the LNU library, but if
you search online you should find many examples and guides, like e.g. {color{green}cite{WEBSITE:2}}
on {color{green}cite{WEBSITE:3}}, or extensive information on {color{green}cite{WEBSITE:1}}.


section{How to submit} 

label{secondSection}

When you compose and compile the final version of this document, you should upload a .zip file on the course moodle that contains:

begin{enumerate}
    item the compiled .pdf file;  
    item   the source files (.tex, .bib, and the image file).
end{enumerate}
The filename template for these files, besides the image file, should be:

begin{itemize}
    item 4ME301-LaTeX-[yoursurname].xxx 
end{itemize}
end{document}

How can i use ref and hyperref or hyperlink to make the number of the section clickable and which are more suitable to use hyperref or hyperlink?

I used both of them and get the same error