How can I fix this error?

Just FYI the following code won't provide the same graph has the step function, it just plots the function having s as the variable, the step fuction just works for proper systems (n poles >= n zeros).

"Impulse response":

syms s
TFC=evalc('tf([1 1 1 1],[1 1])');
[a b] = strread(TFC, '%s %s', 'delimiter',char(10));
num=sym(char(a(2)));
den=sym(char(a(3)));
ezplot(num/den,[0 100])

"Step response"

syms s 
TFC=evalc('tf([1 1 1 1],[1 1 0])');
[a b] = strread(TFC, '%s %s', 'delimiter',char(10)); 
num=sym(char(a(2))); 
den=sym(char(a(3))); 
ezplot(num/den,[0 100])
The scales aren't the same like you get with the step function.