There is some serious TeX slaying and package orgy happening which is normal if you don't have a lot of TeX experience. I was like this too. Many of these stuff is for articles. So avoid loading these packages. For example, the reference stuff Lemma, Conjecture bla bla you don't need at all. Because these are slides if you really have to use cleveref
package but please don't.
So randomly removing stuff until it compiles gives me this.
\documentclass[18pt,xcolor=table]{beamer}
%\usepackage {bbm}
%\usepackage {textpos}
\usepackage{tikz}
%\usepackage{amscd}
\usepackage{amsfonts}
%\usepackage{graphicx}%
%\usepackage{fancyhdr}
%\usepackage{fullpage}
\usepackage{amsmath,amssymb,amsthm}
%\usepackage{enumerate}
%\usepackage{stmaryrd,mathrsfs}
%\usepackage{prettyref,hyperref}
\usetikzlibrary{matrix,calc,3d}
%
%[custom math-mode commands, omitted for brevity]
%
%\newcommand{\drawbox}[1]{\begin{array}{|l|}\hline #1 \\
%\hline\end{array}}
%
%
%\newtheoremstyle{thmstyle}{1em}{1.5em}{}{.7em}{\bf}{:}{0.5em}{}
%\theoremstyle{thmstyle} \newtheorem{thm}{Theorem}[section]
%%\newtheorem{theorem}{Theorem} \newtheorem{conjecture}{Conjecture}
%\newtheorem{project}{Project} \newtheorem{defn}[thm]{Definition}
%\newtheorem{cor}[thm]{Corollary} \newtheorem{lem}[thm]{Lemma}
%\newtheorem{rem}[thm]{Remark} \newtheorem{ex}[thm]{Example}
%\newtheorem{prop}[thm]{Proposition}
%\newtheorem{ithm}{Theorem}[section] \newtheorem{conj}[thm]{Conjecture}
%\newtheorem{question}{Question}
\definecolor{utorange}{RGB}{203,96,21}
\definecolor{utblack}{RGB}{99,102,106}
\definecolor{utbrown}{RGB}{110,98,89}
\definecolor{utsecbrown}{RGB}{217,200,158}
\definecolor{utsecgreen}{RGB}{208,222,187}
\definecolor{utsecblue}{RGB}{127,169,174}
\definecolor{utgray}{HTML}{B2BEB5}
\mode<presentation> { % \usetheme{Pittsburgh} %
\usetheme{Boadilla}
\usefonttheme[onlymath]{serif}
\setbeamercovered{invisible}
\setbeamertemplate{navigation symbols}{}
% Color Theme
\setbeamercolor{normal text}{bg=white,fg=utblack}
\setbeamercolor{structure}{bg=utgray,fg=utorange}
\setbeamercolor{alerted text}{fg=red!85!black}
\setbeamercolor{item projected}{use=item,fg=black,bg=item.fg!35}
\setbeamercolor*{palette primary}{use=structure,fg=white,bg=utorange}
\setbeamercolor*{palette secondary}{use=structure,bg=utsecbrown}
\setbeamercolor*{palette tertiary}{use=structure,bg=utsecgreen}
\setbeamercolor*{palette quaternary}{use=structure,fg=structure.fg,bg=utsecblue}
\setbeamercolor*{frametitle}{use=structure,fg=utorange,bg=utsecbrown}
\setbeamercolor*{framesubtitle}{fg=utbrown}
\setbeamercolor*{block title}{parent=structure,fg=black,bg=utsecgreen}
\setbeamercolor*{block body}{fg=black,bg=utblack!10}
\setbeamercolor*{block title alerted}{parent=alerted text,bg=black!15}
\setbeamercolor*{block title example}{parent=example text,bg=black!15}
\setbeamerfont{framesubtitle}{size=\small}
}
\hypersetup{pdfpagemode=FullScreen}
%\usepackage[orientation=landscape,size=custom,width=16,height=9.75,scale=0.5,debug]{beamerposter}
\usepackage[orientation=landscape,size=custom,width=16,height=9,scale=0.5,debug]{beamerposter}
\makeatletter
\setbeamertemplate{footline}{\leavevmode%
\hbox{%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,center]{author in head/foot}%
\usebeamerfont{author in head/foot}\insertshortauthor%~~\beamer@ifempty{\insertshortinstitute}{}{(\insertshortinstitute)}
\end{beamercolorbox}%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,center]{title in head/foot}%
\usebeamerfont{title in head/foot}\insertshorttitle%
\end{beamercolorbox}%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,right]{date in head/foot}%
\usebeamerfont{date in head/foot}\insertshortdate{}\hspace*{2em}%
\insertframenumber{} / \inserttotalframenumber\hspace*{2ex}%
\end{beamercolorbox}}%
\vskip0pt%
}
\makeatother
\pgfdeclareimage[height=1.0cm]{utbig}{logos/UTWordmark}
\pgfdeclareimage[height=0.6cm]{ut}{logos/UTWordmark}
\pgfdeclareimage[height=1.5cm]{sclogo}{logos/SC12} %
\pgfdeclareimage[height=1.0cm]{scsmall}{logos/SC12}
\title{title} %
\subtitle{If you have one}
\author[author]{\underline{author}}
\institute{institute \\ \mbox{} \\ \pgfuseimage{utbig}}
\date[date]{\pgfuseimage{sclogo} \\ \today}
\begin{document}
\begin{frame}{A}
\end{frame}
\end{document}
Best Answer
I solved this problem by adding this in preamble: