First ‘f’ does not exist in your function because you do not define it in your code, or pass it as an argument.
Second, the subs function only works in the Symbolic Math Toolbox.
Try this instead (assuming that ‘f’ and ‘fun’ are the same):
Approximate_Value = (h/2)*(fun(a)+fun(a+(N-1)*h)+sum(h*fun(1:(N-1))));
Assuming that ‘a’, ‘h’, and ‘N’ are scalars, that should work.