The class 'sym' contains a parse error, cannot be found on MATLAB's search path

Hi

i have the code blow

%% The Quadruple Tank

%%
clc 
clear all ; close all ;

%% define parameters 

A1 = 28 ;  A3 = 28;
A2 = 32; A4 = 32;

a1 = 0.071; a3 = 0.071;
a2 = 0.057 ; a4 = 0.057;

kc = 0.5;
g = 981 ;
k1 = 2.9 ; k2 = 2.9;

h1 = 13.64 ; 
h2 = 16.55;
h3 = 1.91;
h4 = 1.77;

T1 = (A1/a1)*sqrt((2*h1)/g);
T2 = (A2/a2)*sqrt((2*h2)/g);
T3 = (A3/a3)*sqrt((2*h3)/g);
T4 = (A4/a4)*sqrt((2*h4)/g);
%% form Matrix
landa1 = 0.7 ;
landa2 = 0.8;

s = tf('s');

g11 = (landa1*T1*k1*kc)/(A1*(s*T1+1));

g12 = ((1-landa2)*T1*k2*kc)/(A1*(s*T1+1)*(s*T3+1));

g21 = ((1-landa1)*T2*k1*kc)/(A2*(s*T2+1)*(s*T4+1));

g22  = (landa2*T2*k2*kc)/(A2*(s*T2+1));

G = [g11 g12; g21 g22]


gershband(G)

i use three other m files to have answer for gersh band

at first i have answers but after i close my matlab and restart it it has error :

The class 'sym' contains a parse error, cannot be found on MATLAB's search path,

or is shadowed by another file with the same name.

what should i do ?

please help me

thanks very much