Undefined funcrion or variable 't'

hi i'm new in matlab , I have this error Undefined function or variable 't'.

function[sys,x0,str,ts]=reactor_sfun(t,x,u,flag,ks,koh,qin,v,fw,fr,nuh,bh,yh,f,k,ssat)

switch flag,

case 0,

[sys,x0,str,ts]=mdlInitializeSizes(t,x,u,ks,koh,qin,v,fw,fr,nuh,bh,yh,f,k,ssat);

case 1,

sys=mdlDerivatives(t,x,u,ks,koh,qin,v,fw,fr,nuh,bh,yh,f,k,ssat);

case 3,

sys=mdlOutputs(t,x,u,ks,koh,qin,v,fw,fr,nuh,bh,yh,f,k,ssat);

case 4,

sys=mdlGetTimeofNextVarHit( t,x,u,ks,koh,qin,v,fw,fr,nuh,bh,yh,f,k,ssat);

case 9,

sys=mdlTerminate(t,x,u,ks,koh,qin,v,fw,fr,nuh,bh,yh,f,k,ssat);

otherwise

DAStudio.error(['unhandledFlag=', num2str(flag)]);

end

function[sys,x0,str,ts]=mdlInitializeSizes(ks,koh,qin,v,fw,fr,nuh,bh,yh,f,k,ssat)

str=[];

ts=[0 0];

sizes=simsizes;

sizes.NumContStates=3;

sizes.NumDiscStates=0;

sizes.NumOutputs=3;

sizes.NumInputs=3;

sizes.DirFeedthrough=0;

sizes.NumSampleTimes=1;

sys=simsizes(sizes);

x0=[5;4.2;3];%conditions initiales

function sys=mdlDerivatives(t,x,u,Ks,Koh,qin,v,fw,fr,nuh,bh,Yh,f,k,Ssat)

Xbh=x(1);

Ss=x(2);

So=x(3);

Xbhin=u(1);

Sin=u(2);

qa=u(3);

z1=(Ss/(ks+Ss))*(So/(koh+So));

z2=qin/v

dXbh=z2*Xbhin-z2*((fw*(1+fr))/(fr+fw))*Xbh+nuh*z1*Xbh-bh*Xbh;

dSs=z2*(Sin-Ss)+(nuh/Yh)*z1*Xbh-(1-f)*bh*Xbh;

dSo=-z2*So+((Yh-1)/Yh)*nuh*z1*Xbh-K*qa*(Ssat-So);

sys=[dXbh;dSs;dSo];

function sys=mdlOutputs(t,x,u)

Xbh=x(1);

Ss=x(2);

So=x(3);

sys=[Xbh;Ss;So];

function sys=mdlGetTimeofNextVarHit(t,x,u)

sampleTime=1;

syst=t+sampleTime;

function sys=mdlTerminate(t,x,u)

sys=[];