Your quoting of the error message is misleading, the actual error is
! Undefined control sequence.
<argument> \@chairmanname
\\ \@chairmantitle
l.91 \maketitle
The undefined command is always the last token on the second line so \@chairmanname the later lines show the context where the error was detected which is \maketitle.
Note the comments in the file say
%% Make the titlepage based on the above information. If you need
%% something special and can't use the standard form, you can specify
%% the exact text of the titlepage yourself. Put it in a titlepage
%% environment and leave blank lines where you want vertical space.
%% The spaces will be adjusted to fill the entire page. The dotted
%% lines for the signatures are made with the \signature command.
So what you are intended to do is use \begin{titlepage} not \maketitle
alternatively redefine \maketitle the original is:
\def\maketitle{\begin{titlepage}
\large
{\def\baselinestretch{1.2}\Large\bf \choosecase{\@title} \par}
by\par
{\Large \choosecase{\@author}}
\par
\@prevdegrees
\par
\choosecase{Submitted to the} \choosecase{\@department} \\
\choosecase{in partial fulfillment of the requirements for the}
\choosecase{\@degreeword}
\choosecase{of}
\par
\choosecase{\@degree}
\par
at the
\par\MIT\par
\@degreemonth\ \@degreeyear
\par
\@copyrightnotice
\par
\vskip 3\baselineskip
\signature{Author}{\@department \\ \@thesisdate}
\par
\vfill
\unvbox\@titlesupervisor
\par
\vfill
\signature{Accepted by}{\@chairmanname \\ \@chairmantitle}
\vfill
\end{titlepage}}
so if you remove
\signature{Accepted by}{\@chairmanname \\ \@chairmantitle}
The error will go.
