documentclass[border=2pt]{standalone}
    usepackage[utf8]{inputenc}

    usepackage{graphics}
    usepackage{tikz,pgfplots}
    usetikzlibrary{math} % Pour evaluate
    usetikzlibrary{calc} % Pour postionnement relatif

    newcommand*{addZsupaUn}[2]{
        addplot[#2,samples=60,domain=0:Ttrace,thick=3pt,ultra thick,
            evaluate={  Tun = (1 / ( 6 *(#1-sqrt(#1^2 -1)) ));
                    Tdeux = (1 / ( 6 *(#1+sqrt(#1^2 -1)) )); }
            ] {  K * (1 - ( 1/( Tun - Tdeux ) )*( Tun *exp(-x/ Tun ) - Tdeux *exp(-x/ Tdeux ) ))  };
    }

    %newcommand*{LegendFlecheSup}[5]{
    %   coordinate[evaluate={  Tun = (1 / ( 6 *(#1-sqrt(#1^2 -1)) ));
    %               Tdeux = (1 / ( 6 *(#1+sqrt(#1^2 -1)) )); }
    %       ] (C2) at (axis cs:#2, {  K * (1 - ( 1/( Tun - Tdeux ) )*( Tun *exp(-#2/ Tun ) - Tdeux *exp(-#2/ Tdeux ) ))  }) ;
    %   draw node[anchor=#3] (#4) at #5 {$z =#2$};
    %   draw[fleche] (C2) -- (#4.west) ;
    %}

    newcommand*{LegendFlecheSup}[5]{
        coordinate (C2) at (axis cs:#2, {  K * (1 - ( 1/( 0.622 - 0.04466 ) )*( 0.622 *exp(-#2/ 0.622 ) - 0.04466 *exp(-#2/ 0.04466 ) ))  }) ;
        draw node[anchor=#3] (#4) at #5 {$z =#2$};
        draw[fleche] (C2) -- (#4.west) ;
    }

    tikzset{ fleche/.style={<-,>=latex,line width=0.2mm,shorten <=-0.02cm} }

    begin{document}

    begin{tikzpicture}

    defK{2}
    defw{(3.14159*2)}
    %defw{(3.14159*2/Ttrace)/sqrt(1-z^2)}
    defTtrace{3}

    begin{axis}[axis x line=bottom, axis y line = left ,
    width=13cm, height=axisdefaultheight,
    ymax=1.55*K,%axis lines=none,
    ytick=empty,extra y ticks={K}, extra y tick labels={},extra y tick style={grid=major},
    %xtick=empty,
    ]

    addplot[samples=2,domain=0:(Ttrace+0.1),thick=1pt] { K };

    % z > 1 -----
    addZsupaUn{2}{blue,densely dotted}
    addZsupaUn{4}{blue,densely dotted}

    LegendFlecheSup{4}{0.6}{west}{C3}{($(C2)+(axis cs:1,-0.5)$)}
    % LegendFlecheSup{2}{0.6}{south}{C4}{(C3.north)}

    end{axis}
    end{tikzpicture}


    end{document}

! Undefined control sequence.
<argument> axis cs:0.6, { K * (1 - ( 1/( Tun
- Tdeux ) )*( Tun *exp(-0.6...
l.55 end{axis}

newcommand*{LegendFlecheSup}[5]{
        pgfmathparse{(1 / ( 6 *(#1-sqrt(#1^2 -1)) ))} pgfmathresult letTunpgfmathresult
        pgfmathparse{(1 / ( 6 *(#1+sqrt(#1^2 -1)) ))} pgfmathresult letTdeuxpgfmathresult
        coordinate (C2) at (axis cs:#2, {  K * (1 - ( 1/( Tun - Tdeux ) )*( Tun *exp(-#2/ Tun )
                - Tdeux *exp(-#2/ Tdeux ) ))  }) ;
        draw node[anchor=#3] (#4) at #5 {$z =#2$};
        draw[fleche] (C2) -- (#4.west) ;
    }