Is there a way to remove this part in Latex Beamer?

The latex code is as in this link: https://publish.obsidian.md/jayjeong/stackoverflow+question+230531

Thank you so much for your help!

You can remove the headline with \setbeamertemplate{headline}{}.

That being said, you should really clean up your preamble. You load a lot of unnecessary packages, you load some of them multiple times and you load package which are incompatible with beamer.

\PassOptionsToPackage{force}{filehook}
\documentclass[usenames,dvipsnames]{beamer}
\usepackage[USenglish]{babel}
\usepackage[utf8]{inputenc}  
%\usepackage{graphicx}
\usepackage[export]{adjustbox}
\usepackage{ragged2e}
\usepackage{multirow}
\usepackage{hhline}
\usepackage{booktabs,caption}
\usepackage{parskip}
%\usepackage{amsmath}
\usepackage{scrextend}
\usepackage{blindtext}
%\usepackage[bottom, hang, flushmargin]{footmisc} 
\usepackage{lscape}
%\usepackage{setspace}
%\usepackage{hhline}
\usepackage[referable]{threeparttablex}
%\usepackage{booktabs}
\newcommand{\lvl}{~~~}
%\usepackage{caption}
\usepackage{subcaption}
\captionsetup{compatibility=false}
%\usepackage{threeparttable}
\usepackage{ragged2e}
%\usepackage{amsmath}
\usepackage{comment}
\setbeamercovered{transparent} 
%\usepackage{amsmath}
\usepackage{mathptmx} 
\mathchardef\mhyphen="2D

\usepackage{dcolumn,
%booktabs
}
\newcolumntype{L}[1]{>{\raggedleft\arraybackslash}m{#1}}
\newcolumntype{C}[1]{>{\centering\arraybackslash}m{#1}}
\newcolumntype{R}[1]{>{\raggedright\arraybackslash}m{#1}}
\newcolumntype{d}[1]{D{.}{.}{#1}}

\setbeamertemplate{itemize/enumerate body begin}{\footnotesize}
\setbeamertemplate{itemize/enumerate subbody begin}{\footnotesize}

\usepackage{sansmathaccent}
\pdfmapfile{+sansmathaccent.map}

%\usepackage{xcolor}
\definecolor{olive}{rgb}{0.3, 0.4, .1}
\definecolor{fore}{RGB}{249,242,215}
\definecolor{back}{RGB}{51,51,51}
\definecolor{title}{RGB}{255,0,90}
\definecolor{dgreen}{rgb}{0.,0.6,0.}
\definecolor{gold}{rgb}{1.,0.84,0.}
\definecolor{JungleGreen}{cmyk}{0.99,0,0.52,0}
\definecolor{BlueGreen}{cmyk}{0.85,0,0.33,0}
\definecolor{RawSienna}{cmyk}{0,0.72,1,0.45}
\definecolor{Magenta}{cmyk}{0,1,0,0}
\definecolor{gray}{gray}{0.95}
\definecolor{charcoal}{rgb}{0.21, 0.27, 0.31}
\definecolor{uclablue}{rgb}{0.33, 0.41, 0.58}

\definecolor{STAblue2}{RGB}{0, 83, 155}
\definecolor{STAblue1}{RGB}{0, 125, 197}
%\definecolor{STAblue2}{RGB}{0, 174, 239}

\setbeamerfont{author}{size=\small}
\setbeamerfont{institute}{size=\footnotesize}
\setbeamerfont{date}{size=\footnotesize}

%\setbeamertemplate{section in toc}[sections numbered]
%\setbeamertemplate{subsection in toc}[subsections numbered]
%\setbeamertemplate{section in toc}[ball unnumbered]
%\setbeamertemplate{subsection in toc}[ball unnumbered]

\usepackage{outlines}
\def\BO{\begin{outline}} \def\EO{\end{outline}}

\usepackage{natbib}
\bibliographystyle{abbrvnat}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\usetheme{Copenhagen}
\useoutertheme{miniframes}
\useinnertheme{circles}
%\usecolortheme{dove}
%\usecolortheme[named=STAblue2]{structure}

\setbeamercolor{palette primary}{bg=white,fg=uclablue}
%\setbeamercolor{palette secondary}{bg=white,fg=white}
\setbeamercolor{palette tertiary}{bg=uclablue,fg=white}
%\setbeamercolor{palette quaternary}{bg=STAblue1,fg=white}
\setbeamercolor{structure}{fg=uclablue} % itemize, enumerate, etc
\setbeamercolor{section in toc}{fg=uclablue} % TOC sections


% Override palette coloring with secondary
%\setbeamertemplate{footline}[frame number]
\setbeamercolor{subsection in head/foot}{bg=gray,fg=black}
\setbeamertemplate{page number in head/foot}[totalframenumber]
\setbeamertemplate{frametitle}[default][center]
\setbeamercolor{frametitle}{bg=white,fg=black}
\setbeamercolor{title}{bg=white,fg=black}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\setbeamertemplate{headline}{}

\begin{document}

\section{title}
\begin{frame}
content...
\end{frame}

\end{document}