Hi,
The step function that is used only accepts Dynamic System Models as input. One example is tf which you are trying to use. Replacing the syms s k with s = tf(‘s’) in your code clear out the error. But again limit function is only valid for symbolic expression. So, you may choose what you prefer to do.
This works fine if you want to use step.
s = tf('s');
a = 1;
b = -5/s;
K = a + b;
G = 6*s + 12/s^3 + 8*s^2 + 19*s + 12;
F = 1;
Error = (K*G/(1+K*G*F));
step(Error);
If you want to use limit, then the code given in the question works fine.
