One chinese character won´t appear in pdf

If you can download Google's Noto Serif CJK and Noto Sans CJK font families, you may want to try the following version, labelled "version 5", which runs under both XeLaTeX and LuaLaTeX. The Noto Serif CJK TC font family provides 6 font weights (from "Extra Light" to "Black"), and the Noto Sans CJK TC font family provides 7 font weights (from "Thin" to "Black").

 % Version 5
documentclass{article}
usepackage[english,ngerman]{babel}

usepackage{fontspec}
setmainfont{Noto Serif CJK TC} 
setsansfont{Noto Sans CJK TC}[Scale=MatchLowercase]

begin{document}
    Hello World. 
    waijiaquan, 外家拳.
    neijiaquan, 內家拳.
    
sffamily
    Hello World. 
    waijiaquan, 外家拳.
    neijiaquan, 內家拳.
end{document}

With SimSun (which I got with Windows 10) your chars are all there too

documentclass{article}
usepackage{fontspec}
setmainfont{SimSun}
begin{document}
waijiaquan, 外家拳
neijiaquan, 內家拳
end{document}

Your font is missing that character. You can add the command tracinglostchars=2 to print a warning message on the console, instead of silently printing it to the .log file.

For this kind of multilingual document, with only short phrases in Chinese, I would recommend Babel. This MWE works on LuaLaTeX.

documentclass{article}
tracinglostchars=2 % Warn if a character is missing from the font!
usepackage[english, ngerman]{babel}
usepackage{fontspec}

babelprovide[import=zh-Hans, onchar=ids fonts]{chinese} % Or zh-Hant for Traditional.
defaultfontfeatures{ Scale=MatchUppercase, Ligatures=TeX }
babelfont{rm}
          [Scale=1.0, Ligatures={Common,Discretionary}]{New Computer Modern Book}
% From: https://www.google.com/get/noto/#serif-hans or https://www.ctan.org/pkg/notocjksc
babelfont[chinese]{rm}
          {Noto Serif CJK SC}
% Set up babelfont{sf} and babelfont{tt} here, if needed.

babeltags{chinese=chinese}
babeltags{english=english}
babeltags{ngerman=german}

begin{document}
waijiaquan, 外家拳
neijiaquan, 內家拳
end{document}

This will get you German captions and hyphenation, but automatically switch when you start typing Chinese. It also lets you switch languages with textenglish{...}, begin{english}, and so on.

To get the document to compile on XeLaTeX, remove the onchar=ids fonts option and wrap all Chinese text in textchinese{外家拳}. This is also compatible with polyglossia.

The ctex package, which is more recent than XeCJK and also works on other engines, provides two ways to select a different font: fontset= and setCJKmainfont (also setCJKsansfont and setCJKmonofont).

documentclass{article}
tracinglostchars=2 % Warn if a character is missing from the font!
usepackage[UTF8, fontset=none]{ctex}

% From: https://www.google.com/get/noto/#serif-hans or https://www.ctan.org/pkg/notocjksc
setCJKmainfont[Scale=MatchUppercase]{Noto Serif CJK SC}

begin{document}
waijiaquan, 外家拳
neijiaquan, 內家拳
end{document}