the following
% file testbismon.tex
%% see https://github.com/bstarynk/bismon/ for more about Bismon
\documentclass[11pt,a4paper,svgnames]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8x]{inputenc}
\usepackage{alltt}
% https://tex.stackexchange.com/a/342804/42406
%\usepackage{textcomp}
\usepackage{moreverb}
\usepackage{fancyvrb}
\usepackage{fancyhdr}
\usepackage{fancybox}
\usepackage{epsfig}
\usepackage{makeidx}
\usepackage{float}
\usepackage{xcolor}
\usepackage{hyperref}
\usepackage{moreverb}
\usepackage{multirow}
\usepackage{boxedminipage}
\usepackage[square]{natbib}
\usepackage{charter}
%\usepackage{inconsolata}
\usepackage{listings}
\usepackage{relsize}
\usepackage{verbatimbox}
\usepackage{filecontents}
\usepackage{catchfile}
\usepackage{lastpage}
\usepackage{stmaryrd}
\usepackage{ucs}
\usepackage{stix}
\usepackage{newunicodechar}
% bigfoot enables \verb in footnotes
\usepackage{bigfoot}
\usepackage[a4paper, margin=2cm]{geometry}
\newcommand{\bmemail}[1]{{\href{mailto:#1}{\texttt{\textbf{#1}}}}}
\newcommand{\bmurl}[1]{{\href{#1}{\texttt{\textbf{#1}}}}}
% see also http://www.sascha-frank.com/Arrow/latex-arrows.html and
% https://ctan.math.illinois.edu/macros/latex/contrib/newunicodechar/newunicodechar.pdf
% and https://tex.stackexchange.com/a/238135/42406
\newunicodechar{↳}{\rotatebox[origin=c]{180}{$\Lsh$}}
\begin{document}
\section{the test}
\begin{alltt}
the U+21B3 character is {\large ↳} \textit{DOWNWARDS ARROW WITH TIP RIGHTWARDS}
\end{alltt}
\end{document}
does not work on my Debian/Sid machine since pdflatex testbismon.tex gets the following error:
...
(/usr/share/texlive/texmf-dist/tex/latex/ucs/data/uni-33.def)
(/usr/share/texlive/texmf-dist/tex/latex/ucs/data/uninames.dat)
(/usr/share/texlive/texmf-dist/tex/latex/ucs/data/uni-33.def)
! Package ucs Error: Unknown Unicode character 8627 = U+21B3,
(ucs) possibly declared in uni-33.def.
(ucs) Type H to see if it is available with options.
See the ucs package documentation for explanation.
Type H <return> for immediate help.
...
l.50 the U+21B3 character is {\large ↳
} \textit{DOWNWARDS ARROW WITH TIP R...
?
Why does that error happens? How to get rid of it?
My above LaTeX code was inspired by this answer. I am using:
pdfTeX 3.14159265-2.6-1.40.20 (TeX Live 2019/Debian)
kpathsea version 6.3.1
and typing my LaTeX files using emacs 26.

newunicodecharrequires not to useutf8x. Change the option toutf8(or remove the call toinputencaltogether). Also remove\usepackage{ucs}. – egreg Jul 22 '19 at 14:18