Some Chinese characters can no show using ctex

The default fontset (fandol) loaded by ctex and xeCJK may not have glyphs for many archaic Chinese characters. If you're compiling this on Overleaf, you can use the fontset=ubuntu option to load Noto Serif CJK SC:

usepackage[UTF8,fontset=ubuntu]{ctex}

Or if you prefer to load the font explicitly:

usepackage{ctex}
setCJKmainfont{Noto Serif CJK SC} % or some other font that has the glyph

I’m going to expand on imnothere’s fine answer with a few best practices.

documentclass[a4paper]{article}
tracinglostchars=2
usepackage[UTF8, fontset=none]{ctex}

defaultfontfeatures{ Scale=MatchUppercase,
                      Ligatures=TeX,
                      Renderer=HarfBuzz }

%% Noto CJK fonts available at
%%  https://www.google.com/get/noto/help/cjk/
setmainfont{Latin Modern Roman}[Scale=1.0]
setCJKmainfont{Noto Serif CJK SC}
setCJKsansfont{Noto Sans CJK SC}
setCJKmonofont{Noto Sans Mono CJK SC}

begin{document}
珅
end{document}

• You want tracinglostchars=2 so that TeX will give you a warning when your font is missing a character. Without this command, the warning will be silently buried in the middle of your .log file.
• In LuaLaTeX as of 2020, you need to load your CJK fonts with the Renderer=HarfBuzz, or you will get an out-of-memory error. XeTeX uses HarfBuzz by default and works fine. XeLaTeX should give you a benign warning if it sees Renderer=.
• You probably want a Scale= option if you are mixing different fonts.
• You might wish to set sans-serif and monospace fonts as well.
• If you would be overloading the font set from the package anyway, you can skip loading one with fontset=none.