\documentclass[10pt]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[french]{babel}
\usepackage{pstricks}
\usepackage{graphicx,color}
\usepackage[a4paper,left=15mm,right=15mm,top=18mm,bottom=20mm]{geometry}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{mathrsfs}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\usepackage{mathtools}
\DecimalMathComma
\begin{document}
\begingroup
\centering
\begin{tikzpicture}
\begin{axis}[
xlabel={rang de l'année $x_{i}$},
ylabel={salaire net annuel moyen (en euros) $y_{i}$},
axis lines=left,grid=major,xmin=0,xmax=20,ymin=22000,ymax=33000,
scaled ticks=false,
width=0.90\textwidth,
]
\addplot [mark size=2pt,mark=*,only marks,]coordinates {(1,22443)(2,22809)(3,23527)(4,24287)(5,24626)(6,25139)(7,25735)(8,26106)};
\addplot [black,mark size=0.1pt,mark=*]coordinates {(1,22440)(20,32720)};
\draw[red,ultra thick] (axis cs:14,0) |- (axis cs:0,29473.68421);
\end{axis}
\end{tikzpicture}
\endgroup
\end{document}
soit le salaire net annuel est faible, soit votre séparateur des milliers est incorrect...