Error message regarding Midpoint Method

As part of a group project we have a system of 2 non linear differential equations and we have to draw the S=S(t) , I=I(t) graphic using the midpoint method and I'm getting the following error:

"Error in inline expression ==> matrix([[-(I*S)/1000], [(I*S)/1000 - (3*I)/10]]) Undefined function 'matrix' for input arguments of type 'double'.

Error in inline/subsref (line 23) INLINE_OUT_ = inlineeval(INLINE_INPUTS_, INLINE_OBJ_.inputExpr, INLINE_OBJ_.expr);" The system is the following

The code i did is the following:

    % code
clear
syms I S
u=[S;I];
F=[-0.001*S*I;0.001*S*I-0.3*I];
F1=inline(char(F),'I','S');
h=100; %Valores aleatórios
T=100000;
ni=(T/h);
u0=[799;1];
f=zeros(1,2);
k=zeros(1,2);
i=1;
%code
while i<=ni
f(1)=F1(u0(1),u0(2));  
f(2)=F1(u0(1),u0(2));
delta=h*f;
k(1)=F1((u0(1)+h*(1/2)),(u0(2)+h*(1/2)));  
k(2)=F1((u0(1)+h*(1/2)),(u0(2)+h*(1/2)));
u1=u0+h*k;
disp('i:'),disp(i)
disp('u= '),disp(u1)
u0=u1;
i=i+1;
end
  end